gitlab-ci: Define exported global variables once

Don't redefine XDG runtime and gsettings schema paths multiple times, just
export the variables once and reuse them.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/408

.gitlab-ci.yml

@@ -22,11 +22,12 @@ test-mutter:
   artifacts:
     paths:
       - build
+  variables:
+    XDG_RUNTIME_DIR: "$CI_PROJECT_DIR/runtime-dir"
+    GSETTINGS_SCHEMA_DIR: "$CI_PROJECT_DIR/build/data"
   script:
-    - glib-compile-schemas $PWD/build/data
-    - mkdir -m 700 $PWD/runtime-dir
+    - mkdir -m 700 $XDG_RUNTIME_DIR
+    - glib-compile-schemas $GSETTINGS_SCHEMA_DIR
     - >
-      env XDG_RUNTIME_DIR=$PWD/runtime-dir
-      GSETTINGS_SCHEMA_DIR=$PWD/build/data
       dbus-run-session -- xvfb-run -s '+iglx -noreset'
       meson test -C build --no-rebuild -t 10 --verbose --no-stdsplit --wrap catchsegv