[CI] run tests on ubuntu 18.04

Author: jrushlow

.github/workflows/ci.yaml

@@ -15,7 +15,7 @@ jobs:
     coding-standards:
         name: "Coding Standards (${{ matrix.php-version }})"
 
-        runs-on: "ubuntu-latest"
+        runs-on: "ubuntu-18.04"
 
         strategy:
             fail-fast: false
@@ -52,7 +52,7 @@ jobs:
     test:
         name: "PHP ${{ matrix.php-version }} + symfony/skeleton@${{ matrix.symfony-skeleton-stability }}"
 
-        runs-on: ubuntu-latest
+        runs-on: ubuntu-18.04
 
         services:
             mysql: