CI: new workflow for triggering ALCF GitLab CI on Polaris

[shanedsnyder]

Uses: https://github.com/marketplace/actions/trigger-gitlab-ci-through-webhooks

[shanedsnyder]

Okay, all done here (well, there are some smaller unrelated Python issues, but haven't had a close look there yet).

This new workflow will kickoff a series of regression tests on our ALCF GitLab CI instance which builds Darshan and runs test on the Polaris system. Right now, the workflow is configured to run on push events to main and via manual triggers via Github actions tab. Allowing the workflow to be run on every commit to a PR leads to potentially piling up jobs on Gitlab CI (which can take quite some time as its serially running 8 different batch jobs on compute nodes). I also further restricted the workflow to only run on changes to darshan-runtime (and to the workflow itself) -- testing darshan-util stuff on a supercomputer is probably unnecessary.