GH Actions: enclose numeric strings in quotes for robustness

Author: generalmimon

.github/workflows/build.yml

@@ -13,9 +13,9 @@ jobs:
       fail-fast: false
       matrix:
         cpp-standard:
-          - 98
-          - 11
-          - 20
+          - '98'
+          - '11'
+          - '20'
     steps:
       - uses: actions/checkout@v4
       - name: restore