ci: Define global variable MESA_TEMPLATES_COMMIT for ci-templates commit

No functional change.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7902>

.gitlab-ci.yml

@@ -1,5 +1,6 @@
 variables:
   FDO_UPSTREAM_REPO: mesa/mesa
+  MESA_TEMPLATES_COMMIT: &ci-templates-commit 43ac932d5b8ebf43040d302de3c6e734df178384
   CI_PRE_CLONE_SCRIPT: |-
           set -o xtrace
           /usr/bin/wget -q -O- ${CI_PROJECT_URL}/-/raw/${CI_COMMIT_SHA}/.gitlab-ci/download-git-cache.sh | sh -
@@ -7,7 +8,7 @@ variables:
 
 include:
   - project: 'freedesktop/ci-templates'
-    ref: 43ac932d5b8ebf43040d302de3c6e734df178384
+    ref: *ci-templates-commit
     file:
       - '/templates/ci-fairy.yml'
   - project: 'freedesktop/ci-templates'