[CI] Explicitly specify files to be linted

In particular, subprojects in lib/ should be skipped

.gitlab-ci.yml

@@ -20,7 +20,7 @@ ktlint:
     variables: false
   image: kkopper/ktlint:latest
   script:
-    - ktlint
+    - ktlint "src/**/kotlin/**/*.kt" "**/*.kts" '!lib/**'
 
 classes:
   stage: build