ci: Do not append ci-templates commit hash to Windows docker image tag

We're not using the templates for the Windows image.

Fixes needless rebuild of the Windows image when the ci-templates
commit is changed.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9833>

.gitlab-ci.yml

@@ -503,9 +503,10 @@ arm_test:
   extends:
     - .set-image
   variables:
+    MESA_IMAGE: "$CI_REGISTRY_IMAGE/${MESA_IMAGE_PATH}:${MESA_IMAGE_TAG}"
     MESA_IMAGE_PATH: "windows/x64_build"
     MESA_IMAGE_TAG: "2021-03-15-tls"
-    MESA_UPSTREAM_IMAGE: "$CI_REGISTRY/$FDO_UPSTREAM_REPO/$MESA_IMAGE_PATH:${MESA_IMAGE_TAG}--${MESA_TEMPLATES_COMMIT}"
+    MESA_UPSTREAM_IMAGE: "$CI_REGISTRY/$FDO_UPSTREAM_REPO/$MESA_IMAGE_PATH:${MESA_IMAGE_TAG}"
 
 windows_build_vs2019:
   extends: