GitHub Actions: Use Ubuntu 18.04 for older glibc

.github/workflows/linux.yaml

@@ -6,7 +6,7 @@ on:
 jobs:
   build:
     name: ${{ matrix.config.name }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       SHELL: bash
       GTK: ${{ matrix.config.gtk }}