All (running) jobs have a shell.sock unix domain socket in their run directory
(e.g. in [FORREST ENV PATH]/runs/[USER]/[REPO]/[RUNNER_NAME])
that can be used to log into the machine using e.g. socat:
$ socat -,rawer,escape=0x1d UNIX-CONNECT:.../shell.sockNote
You need to press enter to get an initial prompt.
To exit from the shell, press the CTRL-] escape code.