ci: Set XDG_CACHE_HOME to tmpfs for bare-metal runners to avoid NFS.

ci: Set XDG_CACHE_HOME to tmpfs for bare-metal runners to avoid NFS.

We don't want these files shared between builds (it'll get blown away by
the next rsync), and NFS will just increase our latency for hitting the
cache.

Drops a630 gles31 run from 11-17 minutes to 5.5.  Maximum cache size on a
run I've seen is 153M, which it seems we can easily spare.

Fixes: f97acb4bb4b1 ("freedreno/ir3: disk-cache support")
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5998>