fix phpunit_file path for unit tests

Author: schmengler

.github/workflows/unit.yml

@@ -12,7 +12,8 @@ jobs:
       - uses: actions/checkout@v2
       - uses: docker://yireo/github-actions-magento-unit-tests:7.4
         with:
-          phpunit_file: .github/phpunit9-unit.xml
+          # for unit tests, the path is not prefixed with GITHUB_WORKSPACE like for integration tests
+          phpunit_file: vendor/integer-net/magento2-shippingpreselection/.github/phpunit9-unit.xml
         env:
           MAGENTO_VERSION: '2.4.3'
           MODULE_NAME: IntegerNet_ShippingPreselection