Move runs-on under strategy

.github/workflows/build.yml

@@ -4,13 +4,13 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ${{ matrix.os }}
-
     strategy:
       matrix:
         os: [ubuntu-22.04, macos-11]
         compiler: [gcc, clang]
 
+    runs-on: ${{ matrix.os }}
+
     steps:
     - uses: actions/checkout@v3
     - name: Linux setup