execSync just started a second ssh-agent. `['-k']` argument was
treated as options, it didn't have `stdio` set, so stdio was piped and
returned (and ignored).
Thanks to @thommyhh for this contribution!
Unless the `SSH_AUTH_SOCK` is configured explicitly, this change will make the SSH agent use a random file name for the socket. That way, multiple, concurrent SSH agents can be used on non-ephemeral, self-hosted runners.
A new post-action step will automatically clean up the running agent at the end of a job.
Be aware of the possible security implications: Two jobs running on the same runner might be able to access each other's socket and thus access repositories and/or hosts.
