fix github-actions job file

Author: andreasbuhr

.github/workflows/cmake.yml

@@ -6,10 +6,6 @@ env:
   # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)
   BUILD_TYPE: Release
 
-strategy:
-  matrix:
-    compiler: [g++-10, clang]
-
 jobs:
   build:
     # The CMake configure and build commands are platform agnostic and should work equally
@@ -18,6 +14,10 @@ jobs:
     # See: https://docs.github.com/en/actions/configuring-and-managing-workflows/configuring-a-workflow#configuring-a-build-matrix
     runs-on: ubuntu-latest
 
+    strategy:
+      matrix:
+        compiler: [g++-10, clang]
+
     steps:
     - uses: actions/checkout@v2