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

Also skip builds on non-Windows platforms when only Windows build files
have changed.

This should reduce the number of useless builds and the associated
waiting time and chance of spurious failures, freeing resources for
new PRs.

Closes #10742

.github/workflows/fuzz.yml

@@ -9,9 +9,28 @@ on:
     branches:
     - master
     - '*/ci'
+    paths-ignore:
+    - '**/*.md'
+    - '.azure-pipelines.yml'
+    - '.circleci/**'
+    - '.cirrus.yml'
+    - 'appveyor.yml'
+    - 'packages/**'
+    - 'plan9/**'
+    - 'projects/**'
+    - 'winbuild/**'
   pull_request:
     branches:
     - master
+    paths-ignore:
+    - '**/*.md'
+    - '.azure-pipelines.yml'
+    - '.cirrus.yml'
+    - 'appveyor.yml'
+    - 'packages/**'
+    - 'plan9/**'
+    - 'projects/**'
+    - 'winbuild/**'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}