gh-actions: add check for executable source files

Author: nemequ

.github/workflows/ci.yml

@@ -18,6 +18,8 @@ jobs:
       run: git grep -i 'x''1''6''6' && exit 1 || exit 0
     - name: Incorrect assertions in test/
       run: grep -PR '(?<=[^a-zA-Z0-9_])simde_assert_u?int(8|16|32|64)(?>[^a-zA-Z0-9_])' test/ && exit 1 || exit 0
+    - name: Executable sources
+      run: find \( -name '*.c' -o -name '*.h' \) -executable | grep -q '.' && exit 1 || exit 0
 
   x86:
     runs-on: ubuntu-latest