Support for headless use of SecretService in Linux environments

[bpkroth]

Use of secretservice as a backend store doesn't appear to be allowed when there's no DISPLAY environment variable:

git-credential-manager/src/shared/Core/CredentialStore.cs

Line 247 in b62021f

if (!_context.SessionManager.IsDesktopSession)

But even the underlying MSAL library has a headless test for that: https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/main/build/template-test-on-linux.yaml#L61

Can we just remove that check from GCM?

Related:

• Support secretservice in headless environments #565