ci: Manually fetch submodules for style-check CI jobs

This works around GitLab issue
https://gitlab.com/gitlab-org/gitlab/-/issues/391756, which manifests as
the error message:
```
Updating/initializing submodules...
Submodule 'subprojects/gvdb' (https://gitlab-ci-token:[MASKED]@gitlab.gnome.org/GNOME/gvdb.git) registered for path 'subprojects/gvdb'
Synchronizing submodule url for 'subprojects/gvdb'
fatal: not a git repository: subprojects/gvdb/../../.git/modules/subprojects/gvdb
```
on between 1/10 to 1/2 CI runs.

See the GitLab issue for a writeup.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -49,8 +49,18 @@ variables:
     - bash .gitlab-ci/show-execution-environment.sh
     - cp -r $HOME/subprojects/* subprojects/
 
-style-check-advisory:
+# FIXME: Work around https://gitlab.com/gitlab-org/gitlab/-/issues/391756
+.only-default-with-git:
   extends: .only-default
+  before_script:
+   - rm -rf subprojects/gvdb
+   - git submodule update --init --depth 1
+  variables:
+    GIT_SUBMODULE_STRATEGY: "none"
+    GIT_SUBMODULE_DEPTH: 1
+
+style-check-advisory:
+  extends: .only-default-with-git
   image: $DEBIAN_IMAGE
   stage: style-check
   allow_failure: true
@@ -73,15 +83,12 @@ sh-and-py-check:
       - "**/*.sh"
 
 style-check-mandatory:
-  extends: .only-default
+  extends: .only-default-with-git
   image: $DEBIAN_IMAGE
   stage: style-check
   allow_failure: false
   script:
     - .gitlab-ci/run-reuse.sh
-  variables:
-    # The submodules are needed for `reuse lint` to work properly
-    GIT_SUBMODULE_STRATEGY: "normal"
 
 fedora-x86_64:
   extends: