fix(ci): color git diff output

in case prettier finds an error, the git diff is hard to recognize the actual difference so one knows what to fix.
I added the --color switch so the changes are easier recognizable in the actions log

Author: lubber-de

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         yarn add -D github:fomantic/prettier#2.8.1-patched
         && npx prettier --loglevel warn '!dist' '!test/coverage' '!src/semantic.less' '**/*.{css,less,overrides,variables}' --write
         && git restore package.json yarn.lock
-        && git add . -N && git diff --exit-code
+        && git add . -N && git diff --color --exit-code
   test:
     name: Test build process on node ${{ matrix.node-version }}
     runs-on: ubuntu-latest