chore: enable code coverage XML

Author: btry

.github/workflows/continuous-integration.yml

@@ -18,6 +18,9 @@ concurrency:
   group: "${{ github.workflow }}-${{ github.ref }}"
   cancel-in-progress: true
 
+env:
+  PHPUNIT_ARGS: "--coverage-xml=docs/coverage/"
+
 jobs:
   generate-ci-matrix:
     name: "Generate CI matrix"