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>

.gitlab-ci/bare-metal/rootfs-setup.sh

@@ -49,9 +49,8 @@ for var in \
     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:"