Add option for GCM_GPG_PATH environment variable (Linux-only)

[vdye]

Fixes #356.

Changes

• Add support for GCM_GPG_PATH environment variable, used as location of GPG executable if specified (if specified but invalid, exception thrown)
• Updated logic to find GPG executable if explicit path not specified (meant to match behavior in pass, see here)
  • If gpg2 exists, use that; otherwise, use gpg

Testing

• when GCM_GPG_PATH is not set or set to valid GPG executable, GPG is found without error
• when GCM_GPG_PATH is set to "nonsense" (or some other non-file), receive the following error for any git-credential-manager invocation:

  Unhandled exception. System.Exception: GPG executable does not exist with path 'nonsense'
   at Microsoft.Git.CredentialManager.CommandContext.GetGpgPath(IEnvironment environment, IFileSystem fileSystem)
   at Microsoft.Git.CredentialManager.CommandContext..ctor(String appPath)  
   at Microsoft.Git.CredentialManager.Program.Main(String[] args)
  

[mjcheetham]

Looks great! Just a few comments on the wording of the documentation, and a suggestion to add tracing of the resolved GPG path.

[mjcheetham]

By the way you might want to rebase this work on the latest main branch that includes a workaround for the flakey test.