Matrix test using the latest version of XRootD

[turetske]

While Pelican currently uses the latest release of xrootd in all its dev containers, we want to determine if some change in XRootD causes test failures in Pelican before it gets into the xrootd release.

To do this, create a workflow which runs our tests (xrootd-unit and end-to-end), against the latest source code of XRootD. If this test run fails, it should generate a warning that something's gone wrong with XRootD.

These tests should be run as a separate workflow from our current testing branch simply because we don't want failure in this workflow to cause the other tests to stop running. If the main test workflow passes and this workflow fails, then that means the current PR is fine, but something has been changed in XRootD that we should inform their team about, but it doesn't prevent the current PR from merging in.