[BUGFIX] Not working properly on self-hosted runner

* remove fixed default auth socket path
* call `ssh-agent` with auth socket path, if not explicitly given,
  to let the system choose the path
* read SSH_AUTH_SOCKET and SSH_AGENT_PID from `ssh-agent` command output
  and add the as variables

action.yml

@@ -6,7 +6,6 @@ inputs:
         required: true
     ssh-auth-sock:
         description: 'Where to place the SSH Agent auth socket'
-        default: /tmp/ssh-auth.sock
 runs:
     using: 'node12'
     main: 'dist/index.js'