Validate environment URL properly with CI lint

Sorry if this is a duplicate somewhere.

The following config is problematic because ${CI_BUILD_REF_SLUG:0:20} is not a valid syntax, therefore deployment could not be properly created. However, the linter would not complain it:

job:
  environment:
    name: review/$CI_BUILD_REF_NAME
    url: http://example-${CI_BUILD_REF_SLUG:0:20}.example.com
  script:
    - echo "Deploy"

/cc @grzesiek @zj @Konkrad

I didn't check if this applies to the other section. Sorry I am in a rush.