ci: user rules: instead on only: (deprecated)

also check required variables for find-files job

.gitlab-ci.yml

@@ -17,10 +17,12 @@ find-files:
     - echo "commit SHA $LATEST_COMMIT_SHA"
     - |
       echo "working on files... $(git diff-tree --no-commit-id --name-only -r $LATEST_COMMIT_SHA | tee changed-files.txt)"
-  only:
+  rules:
+    if: $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA != null || $CI_COMMIT_SHA != null
     changes:
-      - "*.md"
-      - "**/*.md"
+      paths:
+        - "*.md"
+        - "**/*.md"
   artifacts:
     paths:
       - changed-files.txt
@@ -52,10 +54,11 @@ mdlint:
     - |
       echo "working on files: $(cat changed-files.txt)"
     - ./bin/mdl-wrapper $(cat changed-files.txt)
-  only:
+  rules:
     changes:
-      - "*.md"
-      - "**/*.md"
+      paths:
+        - "*.md"
+        - "**/*.md"
 
 # this will simply run all the time, regardless of which files
 # changed, so it doesn't require the above
@@ -80,10 +83,11 @@ codespell:
     - apt install -qy codespell
   script:
     - codespell $(cat changed-files.txt)
-  only:
+  rules:
     changes:
-      - "*.md"
-      - "**/*.md"
+      paths:
+        - "*.md"
+        - "**/*.md"
 
 codespellall:
   image: