Firebase CLI ignores GOOGLE_APPLICATION_CREDENTIALS and uses missing default service account

### [REQUIRED] Environment info

**firebase-tools:** 14.2.1
**Platform:** macOS

### [REQUIRED] Test case

Running the following commands fails to deploy the extension and ignores the provided custom service account:

```bash
GOOGLE_APPLICATION_CREDENTIALS="./serviceAccount.prod.json" firebase deploy --only extensions --project=
```

or

```bash
firebase deploy --only extensions --project=
```

Need to run without service account created (delete)

### [REQUIRED] Steps to reproduce

1. Remove the default service account
2. Create a custom service account with all necessary permissions.
3. Set `GOOGLE_APPLICATION_CREDENTIALS` to its credentials JSON.
4. Deploy an extension using `firebase deploy --only extensions`.
5. Notice that Firebase ignores the custom account and tries to use the default compute account.

### [REQUIRED] Expected behavior

Firebase CLI should fully respect the `GOOGLE_APPLICATION_CREDENTIALS` variable and use the provided credentials for all operations (IAM, Cloud Functions, etc.).

### [REQUIRED] Actual behavior

The CLI ignores the custom credentials and attempts to use a non-existent default compute account, resulting in the following error:

```
Error: Request to https://cloudresourcemanager.googleapis.com/v1/projects/:setIamPolicy had HTTP Error: 400, Service account -compute@developer.gserviceaccount.com does not exist.
```

Additional console errors:

```json
"ResourceErrorCode":"500",
"ResourceErrorMessage":"Gen1 operation for function ... failed."
...
"code":"404",
"message":"Service account projects/-/serviceAccounts/-compute@developer.gserviceaccount.com was not found."
```

This default service account does **not exist** in my project and **should not be used**.