fix: Fix ubuntu version on CI

Github Actions changed ubuntu-latest to ubuntu-20.04, which update
libxml from 2.9.4 to 2.9.10. This version handles whitespaces
differently and tests were failing.

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
 
         strategy:
             matrix:
-                operating-system: [ubuntu-latest]
+                operating-system: [ubuntu-18.04]
                 php-versions: ['7.3', '7.4']
 
         services: