migrate-secret-alerts breaks as it doesn't pick up command line argument values

## Description

After the [v1.18.0](https://github.com/github/gh-gei/releases/tag/v1.18.0) release the `gh gei migrate-secret-alerts` is not working as expected because the access tokens provided in the cli arguments are not picked up.

It now asks for `GH_PAT` environment variable to be defined which was not necessary before. Once defined it seems that it is used as token for both the `--github-source-pat` and the `--github-target-pat` which is obviously not correct for cases when the _source_ and _target_ repositories are in different organizations where access is provided for each orgs.

## Reproduction Steps

Defining the vars and running the command gives the `GH_PAT environment variable is not set` error
```
SOURCE_ORG=thez-org
SOURCE_REPO=gitlab-foss
SOURCE_PAT=gho_token1
DESTINATION_ORG=theztefan-enterprise-org
DESTINATION_REPO=target
DESTINATION_PAT=ghs_token2

gh gei migrate-secret-alerts \
--source-org "${SOURCE_ORG}" \
--source-repo "${SOURCE_REPO}" \
--target-org "${DESTINATION_ORG}" \
--target-repo "${DESTINATION_REPO}" \
--github-source-pat "${SOURCE_PAT}" \
--github-target-pat "${DESTINATION_PAT}" \
--verbose
[2025-08-08 14:15:24] [INFO] You are running an up-to-date version of the gei CLI [v1.18.0]
[2025-08-08 14:15:24] [INFO] SOURCE ORG: thez-org
[2025-08-08 14:15:24] [INFO] SOURCE REPO: gitlab-foss
[2025-08-08 14:15:24] [INFO] TARGET ORG: theztefan-enterprise-org
[2025-08-08 14:15:24] [INFO] TARGET REPO: target
[2025-08-08 14:15:24] [INFO] GITHUB SOURCE PAT: ***
[2025-08-08 14:15:24] [INFO] GITHUB TARGET PAT: ***
[2025-08-08 14:15:24] [INFO] VERBOSE: true
[2025-08-08 14:15:24] [ERROR] OctoshiftCLI.OctoshiftCliException: GH_PAT environment variable is not set.
at OctoshiftCLI.Services.EnvironmentVariableProvider.GetValue(String name, Boolean throwIfNotFound)
at OctoshiftCLI.Services.EnvironmentVariableProvider.GetSecret(String secretName, Boolean throwIfNotFound)
at OctoshiftCLI.Services.EnvironmentVariableProvider.TargetGithubPersonalAccessToken(Boolean throwIfNotFound)
at OctoshiftCLI.Services.EnvironmentVariableProvider.SourceGithubPersonalAccessToken(Boolean throwIfNotFound)
at OctoshiftCLI.Factories.GithubApiFactory.OctoshiftCLI.Contracts.ISourceGithubApiFactory.Create(String apiUrl, String uploadsUrl, String sourcePersonalAccessToken)
at OctoshiftCLI.GithubEnterpriseImporter.Factories.SecretScanningAlertServiceFactory.Create(String sourceApi, String sourceToken, String targetApi, String targetToken, Boolean sourceApiNoSsl)
at OctoshiftCLI.GithubEnterpriseImporter.Commands.MigrateSecretAlerts.MigrateSecretAlertsCommand.BuildHandler(MigrateSecretAlertsCommandArgs args, IServiceProvider sp)
at OctoshiftCLI.Extensions.CommandExtensions.RunHandler[TArgs,THandler](TArgs args, ServiceProvider sp, CommandBase`2 command)
at OctoshiftCLI.Extensions.CommandExtensions.<>c__DisplayClass1_0`3.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Invocation.AnonymousCommandHandler.InvokeAsync(InvocationContext)
at System.CommandLine.Invocation.InvocationPipeline.<>c__DisplayClass4_0.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass17_0.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass12_0.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass22_0.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass19_0.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<b__18_0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass16_0.<b__0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<b__5_0>d.MoveNext()
--- End of stack trace from previous location ---
at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass8_0.<b__0>d.MoveNext()
```

Setting a GH_PAT and then running the command shows that the GH_PAT is used for both the _source_ and the _target_
```
SOURCE_ORG=thez-org
SOURCE_REPO=gitlab-foss
SOURCE_PAT=gho_token1
DESTINATION_ORG=theztefan-enterprise-org
DESTINATION_REPO=target
DESTINATION_PAT=ghs_token2

export GH_PAT=${SOURCE_PAT}

[2025-08-08 14:24:11] [INFO] You are running an up-to-date version of the gei CLI [v1.18.0]
[2025-08-08 14:24:11] [INFO] SOURCE ORG: thez-org
[2025-08-08 14:24:11] [INFO] SOURCE REPO: gitlab-foss
[2025-08-08 14:24:11] [INFO] TARGET ORG: theztefan-enterprise-org
[2025-08-08 14:24:11] [INFO] TARGET REPO: target
[2025-08-08 14:24:11] [INFO] GITHUB SOURCE PAT: ***
[2025-08-08 14:24:11] [INFO] GITHUB TARGET PAT: ***
[2025-08-08 14:24:11] [INFO] VERBOSE: true
[2025-08-08 14:24:11] [INFO] Migrating Secret Scanning Alerts...
[2025-08-08 14:24:11] [INFO] Migrating Secret Scanning Alerts from 'thez-org/gitlab-foss' to 'theztefan-enterprise-org/target'
[2025-08-08 14:24:11] [DEBUG] HTTP GET: https://api.github.com/repos/thez-org/gitlab-foss/secret-scanning/alerts?per_page=100
[2025-08-08 14:24:11] [DEBUG] GITHUB REQUEST ID: EF3E:3811AC:726241:6A6762:6895EC6B
[2025-08-08 14:24:11] [DEBUG] RESPONSE (OK): [{"number":45,"created_at":"2025-07-29T22:16:20Z","updated_at":"2025-07-29T22:16:20Z","url":"https://api.github.com/repos/thez-org/gitlab-f

--- redacted debug msgs that show secret scanning alert data is fetched correctly from source repo ---

...
```

Getting to the next step where the command should pull alerts from the target repo we see that the calls fail even though the `github-target-pat` is valid which implies that the same `GH_PAT` token is used and not the one provided as a cli argument.

```
[2025-08-08 14:24:18] [DEBUG] HTTP GET: https://api.github.com/repos/theztefan-enterprise-org/target/secret-scanning/alerts?per_page=100
[2025-08-08 14:24:18] [DEBUG] GITHUB REQUEST ID: EF3E:3811AC:72894B:6A8BFC:6895EC72
[2025-08-08 14:24:18] [DEBUG] RESPONSE (NotFound): {"message":"Not Found","documentation_url":"https://docs.github.com/rest/secret-scanning/secret-scanning#list-secret-scanning-alerts-for-a-repository","status":"404"}
[2025-08-08 14:24:18] [DEBUG] [HTTP ERROR 404] System.Net.Http.HttpRequestException: GitHub API error: {"message":"Not Found","documentation_url":"https://docs.github.com/rest/secret-scanning/secret-scanning#list-secret-scanning-alerts-for-a-repository","status":"404"}
---> System.Net.Http.HttpRequestException: Response status code does not indicate success: 404 (Not Found).
at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
at OctoshiftCLI.Services.GithubClient.SendAsync(HttpMethod httpMethod, String url, Object body, HttpStatusCode expectedStatus, Dictionary`2 customHeaders)
--- End of inner exception stack trace ---
at OctoshiftCLI.Services.GithubClient.SendAsync(HttpMethod httpMethod, String url, Object body, HttpStatusCode expectedStatus, Dictionary`2 customHeaders)
at OctoshiftCLI.Services.GithubClient.<>c__DisplayClass20_0.<b__0>d.MoveNext()
...
```