ci: Instruct gitlab to checkout submodules

By default, gitlab skips checking out of submodules. So far this
didn't matter to us, as meson will initialize submodules as part
of setup.

It no longer does for the dist command however, and as we run it
from a separate job where the submodule checkout isn't preserved,
the job breaks with recent meson versions.

Fix this by instructing gitlab to checkout the submodule for all
jobs that need it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2371>

.gitlab-ci.yml

@@ -185,6 +185,8 @@ potfile_js_check:
 build:
   stage: build
   needs: ["check_commit_log"]
+  variables:
+    GIT_SUBMODULE_STRATEGY: normal
   before_script:
     - .gitlab-ci/checkout-mutter.sh
     - meson mutter mutter/build --prefix=/usr
@@ -203,6 +205,7 @@ test:
   stage: test
   needs: ["build"]
   variables:
+    GIT_SUBMODULE_STRATEGY: normal
     XDG_RUNTIME_DIR: "$CI_PROJECT_DIR/runtime-dir"
     NO_AT_BRIDGE: "1"
   before_script:
@@ -225,6 +228,8 @@ test-coverity:
   needs: ["build"]
   stage: analyze
   allow_failure: true
+  variables:
+    GIT_SUBMODULE_STRATEGY: normal
   before_script:
     - meson install -C mutter/build
   script:
@@ -260,6 +265,7 @@ dist:
   variables:
     XDG_RUNTIME_DIR: "$CI_PROJECT_DIR/runtime-dir"
     NO_AT_BRIDGE: "1"
+    GIT_SUBMODULE_STRATEGY: normal
   stage: deploy
   needs: ["build"]
   before_script: