Properly setup github actions due to switch to ubuntu 20.04

Signed-off-by: Julius Härtl <jus@bitgrid.net>

.github/workflows/appbuild.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     strategy:
       matrix:

.github/workflows/phpunit.yml

@@ -66,7 +66,7 @@ jobs:
         with:
           php-version: ${{ matrix.php-versions }}
           tools: phpunit
-          extensions: mbstring, iconv, fileinfo, intl, sqlite, pdo_sqlite, mysql, pdo_mysql, pgsql, pdo_pgsql
+          extensions: mbstring, iconv, fileinfo, intl, sqlite, pdo_sqlite, mysql, pdo_mysql, pgsql, pdo_pgsql, zip, gd
           coverage: none
 
       - name: Set up PHPUnit