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: f97acb4bb4 ("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>

.gitlab-ci/bare-metal/init.sh

@@ -7,9 +7,13 @@ mount -t sysfs none /sys
 mount -t devtmpfs none /dev || echo possibly already mounted
 mkdir -p /dev/pts
 mount -t devpts devpts /dev/pts
+mount -t tmpfs tmpfs /tmp
 
 . /set-job-env-vars.sh
 
+# Store Mesa's disk cache under /tmp, rather than sending it out over NFS.
+export XDG_CACHE_HOME=/tmp
+
 echo "nameserver 8.8.8.8" > /etc/resolv.conf
 
 if sh $BARE_METAL_TEST_SCRIPT; then