[ci] replace obsolete fmt option

.gitlab-ci.yml

@@ -58,7 +58,7 @@ fmt:
     - export PATH="$HOME/.cargo/bin:$PATH"
     - cargo install --force rustfmt-nightly
   script:
-    - cargo fmt -- --write-mode=diff
+    - cargo fmt -- --check
   allow_failure: true
 
 publish:crate: