CI: use bash style variable expansion for the mingw cache key

It doesn't understand the powershell syntax which leads to the cache
key being empty which leads to cache restore being skipped
which leads to slow builds not using ccache.

See https://gitlab.com/gitlab-org/gitlab-runner/-/issues/2747

.gitlab-ci.yml

@@ -104,7 +104,7 @@ release-build:
     - C:\msys64\usr\bin\pacman --noconfirm -Syyuu
     - C:\msys64\usr\bin\bash -lc "bash -x ./.gitlab-ci/test-msys2.sh"
   cache:
-    key: "${env:CI_JOB_NAME}"
+    key: "$CI_JOB_NAME"
     <<: *cache-paths
 
 msys2-mingw32: