ci/llvmpipe: Move most of testing to shared 64-core runners at Google.

ci/llvmpipe: Move most of testing to shared 64-core runners at Google.

These runners are configured to have a single job take up the whole
runner, which means we get to use threads to our hearts content.  The pile
of cores means we don't need to spawn separate jobs to try to load-balance
across fdo's shared runner capacity.  Having dedicated runners means we
won't get our MRs blocked as much waiting on non-Mesa testing happening on
fd.o.

We manage to complete all of this llvmpipe testing in about 6:15.

Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14962>