Make unrelated PRs not rebuild (#375)

(this isn’t tested, and there’s no way for me to know if it works)

.github/workflows/build.yml

@@ -11,6 +11,14 @@ on:
       - 'CMakeLists.txt'
       - '.github/workflows/build.yml'
   pull_request:
+    paths:
+        - 'src/**'
+        - 'docs/**'
+        - 'patches/**'
+        - 'include/**'
+        - 'Makefile'
+        - 'CMakeLists.txt'
+        - '.github/workflows/build.yml'
   workflow_dispatch:
 
 jobs: