Fixes CI script.

Author: Felix "xq" Queißner

.github/workflows/validate.yml

@@ -8,7 +8,13 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest, macos-latest, windows-latest
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
         uses: actions/checkout@v2