Don’t retry EPERM issues relating to git-mirrors

See https://github.com/buildkite/agent/issues/1017#issuecomment-861906324 for context on this. This is a non-recoverable error we should detect and bail on using mirrors when it occurs.

Start with the linked issue comment for the missing context, then locate the git-mirror retry handling in the agent. Confirm how EPERM errors are identified and verify that this case stops mirror retries rather than being treated as recoverable.

Written by the indexing model from the issue text.