CI: Fix rustfmt job.

.gitlab-ci.yml

@@ -125,7 +125,7 @@ rustfmt:
     script:
         - rustc -Vv && cargo -Vv
         - cargo fmt --version
-        - cargo fmt --all -- --write-mode=diff
+        - cargo fmt --all -- --check
 
 # Configure and run clippy on nightly
 # Only fails on errors atm.
@@ -140,4 +140,4 @@ clippy:
         # Force regeneration of gresources regardless of artifacts chage
         - cd hammond-gtk/resources/ && glib-compile-resources --generate resources.xml && cd ../../
         - cargo clippy --all
-    <<: *cargo_cache
+    <<: *cargo_cache