gitlab-ci: fix quoting of variables passed down to bare-metal runners

Eric Anholt identified the issue when merging one of my MRs: the
variable contained words in '`' backticks, which caused them to be
executed by the bare metal runner's shell.

Quote the value printed using bash's shell expansion feature to make
sure anything in the future will be properly quoted.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6389>
diff --git a/.gitlab-ci/bare-metal/rootfs-setup.sh b/.gitlab-ci/bare-metal/rootfs-setup.sh
index 220968a..27a5c40 100644
--- a/.gitlab-ci/bare-metal/rootfs-setup.sh
+++ b/.gitlab-ci/bare-metal/rootfs-setup.sh
@@ -49,9 +49,8 @@
     TU_DEBUG \
     VK_DRIVER \
     ; do
-  val=`echo ${!var} | sed 's|"||g'`
   if [ -n "${!var+x}" ]; then
-    echo "export $var=\"${val}\"" >> $rootfs_dst/set-job-env-vars.sh
+    echo "export $var=${!var@Q}" >> $rootfs_dst/set-job-env-vars.sh
   fi
 done
 echo "Variables passed through:"