ci: Run our manual jobs during the nightly scheduled run.

We have a common pain point with fractional CTS coverage, where the test
list changes on a CTS uprev or board load rebalancing, so you get a
different subset of tests run.  The dev updates the list of xfails (a
pain), but also we end up with xfails left behind that aren't tested any
more and don't reflect reality.

For some drivers (tu, freedreno, zink-anv) we have manual jobs available
for curious devs to look at the current state of the CTS, but without
anyone having to keep the full xfails updated during uprevs, you don't
necessarily know what to do with the results you get on your MR.

So, let's introduce nightly testing for the tests that aren't guaranteed
green by Marge.  With that, Someone (possibly me? sigh) can review the
nightly results and push up updates for full-run xfails so everyone can be
on the same page other than a day or so of delay.  We also have some hope
for automated tooling to do this thanks to what Collabora has been working
on for automated CI uprev MR generation.

TODO:

• @bentiss shall I take over Mesa's nightly pipeline so you don't get spammed with all the fallout of this?
• @daniels What time of day is the lowest usage of the runners? Currently that pipeline is at 4am gmt it looks like, and that seems pretty reasonable as a start time -- north america should definitely have knocked off work by then, and hopefully you're asleep.