CI: don't run CI jobs if only another CI was changed

A few paths were missed in the last commit, as well as a job added since
then.

Followup-to 395b9175

appveyor.yml

@@ -357,6 +357,15 @@ branches:
         - master
         - /\/ci$/
 
+skip_commits:
+  files:
+    - '.azure-pipelines.yml'
+    - '.circleci/**'
+    - '.cirrus.yml'
+    - '.github/**'
+    - 'packages/**'
+    - 'plan9/**'
+
 artifacts:
   - path: '**/curl.exe'
     name: curl