AzureCliCredential drops HOME and creates telemetry files in the current directory

**Describe the bug**

On POSIX systems, `AzureCliCredential` launches `/bin/sh` with a custom child environment containing only `PATH`:

https://github.com/Azure/azure-sdk-for-cpp/blob/main/sdk/identity/azure-identity/src/azure_cli_credential.cpp#L630-L648

This drops Azure CLI environment settings such as `HOME`, `XDG_CACHE_HOME`, and `AZURE_CONFIG_DIR`.

With Azure CLI 2.86.0 and `py-deviceid` 0.1.1, a token request still succeeds because Python can resolve the account's home directory through the user database. However, Azure CLI telemetry reads `HOME` directly. With `HOME` absent, it creates a relative device-ID path under the caller's current working directory:

```text
./None/.cache/Microsoft/DeveloperTools/deviceid
```

The file contains only a random device identifier, not an access token, but a credential call should not pollute an application's working directory. Dropping `AZURE_CONFIG_DIR` can also make a login stored in a non-default Azure CLI config directory unavailable to the credential.

***Exception or Stack Trace***

No exception is raised. Token acquisition succeeds and the filesystem side effect is silent.

**To Reproduce**

1. Sign in with Azure CLI.
2. Run a storage token request with the same PATH-only environment used by the SDK:

```bash
sandbox=$(mktemp -d)
cd "$sandbox"
env -i PATH="$PATH:/usr/bin:/usr/local/bin" \
az account get-access-token \
--output json \
--scope https://storage.azure.com/.default >/dev/null
find "$sandbox" -type f -print
```

3. Observe:

```text
/None/.cache/Microsoft/DeveloperTools/deviceid
```

***Code Snippet***

The same behavior is reached through the SDK:

```cpp
Azure::Identity::AzureCliCredential credential;
Azure::Core::Credentials::TokenRequestContext request;
request.Scopes = {"https://storage.azure.com/.default"};
auto token = credential.GetToken(request, {});
```

**Expected behavior**

The POSIX child environment should retain the allowlisted variables Azure CLI needs to locate its existing user configuration and cache, at minimum:

- `HOME`
- `XDG_CACHE_HOME`, when defined
- `AZURE_CONFIG_DIR`, when defined

`AzureCliCredential` should continue to avoid inheriting the entire parent environment.

**Setup (please complete the following information):**

- OS: Ubuntu 22.04
- IDE: not applicable
- Azure SDK for C++: commit `1c32f92de06445c467715b55860083202194aee4`; behavior also present on `main` as of 2026-08-12
- Azure CLI: 2.86.0
- Azure CLI Core: 2.86.0
- `py-deviceid`: 0.1.1

**Additional context**

The Windows implementation already allowlists `USERPROFILE` so Azure CLI can locate the signed-in user's state. The POSIX implementation currently allowlists only `PATH`.

I can submit a focused PR that propagates the three POSIX variables above and adds an `AzureCliCredential` unit test using the existing synthetic-command test seam.

**Information Checklist**

- [x] Bug Description Added
- [x] Repro Steps Added
- [x] Setup information Added