ci: gcc-4.8 package is only available in Ubuntu 18.04

Author: CohenArthur

.github/workflows/ccpp.yml

@@ -168,7 +168,7 @@ jobs:
 
   build-and-check-gcc-48:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2