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

[Christoph Reiter <creiter src gnome org>]

commit 94202e63395ba591949c4c0218fd8cf6fc3a2b69
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Wed May 6 21:19:52 2020 +0200

    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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 8a80b60435..b228e9c657 100644
--- a/.gitlab-ci.yml
+++ b/.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: