Don't bother with gpg-agent if it isn't present.

The gpg-agent code was trying to run even when gpg-agent wasn't
installed on the host, which resulted in errors like the following:

chmod: /Users/automation/.gnupg/gpg-agent.env: No such file or directory

Now I do a check to make sure the gpg-agent command exists before
trying to futz with it at all.  This will also short circuit the
logic, resulting in quicker startup times when gpg-agent isn't present.