refactor: check vendor_prefixed directory

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>

includes/tests/lint.yml

 .lint: &lint-tmpl
   stage: tests
-  script: find -name '*.php' -not -path '*vendor*' -print0 | xargs -n1 -0 -P16 php -l >/dev/null
+  script:
+    - find -name '*.php' -not -path '*vendor/*' -print0 | xargs -n1 -0 -P16 php -l >/dev/null
 
 lint:8.1:
   <<: *lint-tmpl