Changes how the check for formatting is done.

Now the CI job uses the --check flag provided by cmake_format instead of
the `git diff` check, because git caused some problems in this repo.

.gitlab-ci.yml

@@ -18,8 +18,7 @@ cmake_format:
   before_script: []
   script:
     - sudo apt update && sudo apt install -y python3-pip && pip3 install --user cmake_format pyyaml
-    - env PATH=~/.local/bin:$PATH find . -name "*.cmake" -exec cmake-format -i {} +
-    - git diff --exit-code || (echo "CMake formatting issues found. Please fix them first!"; $(exit 1))
+    - env PATH=~/.local/bin:$PATH find . -name "*.cmake" -exec cmake-format --check {} + || (echo "CMake formatting issues found. Please fix them first!"; $(exit 1))
 
 deps:
   script: