The staging workflow fails to produce independent agent release `.msi` binary

While we were testing out independent agent workflow using the branch in https://github.com/elastic/elastic-stack-installers/pull/281, we noticed that the staging workflow fails work with independent agent release version string.

Upon inspecting the [Buildkite log](https://buildkite.com/elastic/elastic-stack-installers/builds/5557#0190742a-9ef3-42b8-b20c-1504db035c0d), the workflow fails while locating elastic-agent artifact.

```
ElastiBuild: FindPackage#elastic-agent: Starting...
--
  | Searching local directory C:\users\buildkite\esi\bin\in ...
  | Searching Artifacts API for elastic-agent-8.14.0+build202407021002 ...
  | ElastiBuild: FindPackage#elastic-agent: System.Net.Http.HttpRequestException: Response status code does not indicate success: 404 (Not Found).
  | at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
  | at System.Net.Http.HttpClient.GetStreamAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
  | at ElastiBuild.Infra.ArtifactsApi.FindArtifact(String target, Action`1 filterConfiguration) in C:\users\buildkite\esi\src\build\Infra\ArtifactsApi.cs:line 106
  | at ElastiBuild.BullseyeTargets.FindPackageTarget.RunAsync(BuildContext ctx, String targetName) in C:\users\buildkite\esi\src\build\BullseyeTargets\FindPackageTarget.cs:line 69
  | at ElastiBuild.Commands.BuildCommand.<>c__DisplayClass20_0.<b__2>d.MoveNext() in C:\users\buildkite\esi\src\build\Commands\BuildCommand.cs:line 67
  | --- End of stack trace from previous location ---
  | at Bullseye.Internal.ActionTarget.RunAsync(Boolean dryRun, Boolean parallel, Logger log, Func`2 messageOnly)
  | ElastiBuild: FindPackage#elastic-agent: Failed! Response status code does not indicate success: 404 (Not Found). (383 ms)
  | ElastiBuild: ───────────────────────────────────────────────────
  | ElastiBuild: Duration Outcome Target
  | ElastiBuild: ───────────── ───────── ─────────────────────────
  | ElastiBuild: 383 ms 7.5% Failed! FindPackage#elastic-agent
  | ElastiBuild: 4.73 s 92.5% Succeeded BuildBeatPackageCompiler
  | ElastiBuild: ───────────────────────────────────────────────────
  | ElastiBuild: Failed! (BuildInstaller#elastic-agent) (5.12 s)
  | Build staging completed with exit code 0
  |  
```

There could be additional failures in other areas of the workflow. The scope of this issue is to make the workflow support independent agent release.

### Expected Behavior
The pipeline should complete successfully producing a Windows Installer `.MSI` for Independent Agent release.
- The resulting artifact must include independent agent version string suffix.

### Actual Behavior
It fails.

### Steps to reproduce the behaviour
Create a new build at https://buildkite.com/elastic/elastic-stack-installers/builds?branch=pull%2F281%2Fmerge with following environment variables
```
DRA_BRANCH="8.14"
DRA_VERSION="8.14.0+build202407011002"
DRA_WORKFLOW="staging"
MANIFEST_URL="https://staging.elastic.co/independent-agent/8.14.0+build202407021002/manifest-8.14.0+build202407021002.json"
ONLY_AGENT="true"
```