build: clear stalled jobs on POSIX CI hosts

Sometimes, after a cluster or debug test fails, a fixture hangs around
and holds onto a needed port, causing subsequent CI runs to fail. This
adds a command I've been running manually when this occurs. The command
will clear the stalled jobs before a CI run.

PR-URL: https://github.com/nodejs/node/pull/11246


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Josh Gavant <josh.gavant@outlook.com>