CI always failing on tags for phpcs and phsptan

I added a rule in a place that is used by multiple jobs, but I think it might actually be ok, as I'm not sure if we need these jobs to run on tags.
I'd say this is ready for review.

Think we should merge and see if it works.

If we don't want CI to run on tag, should this just be in workflow:rules somewhere?

Alternatively, we could figure out why composer validate fails when run on a tag, because that seems a bit strange.

So the issue is

  COMPOSER_ROOT_VERSION: "${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}${CI_COMMIT_BRANCH}-dev"

For tags neither of those variables are set and this results in the error:

$ COMPOSER_ROOT_VERSION=-dev composer validate

In VersionParser.php line 191:
                                 
  Invalid version string "-dev"                               

I think any job that uses composer is going to fail in the same way. The suggested fix skips any of the lint jobs, but I think the default test job will now try to run? But that will presumably also fail.

Looking at the table at https://docs.gitlab.com/ee/ci/jobs/job_rules.html#run-jobs-only-in-speci...

Maybe if we want to only run on branches we should change

    # Run on commits.
    - if: $CI_PIPELINE_SOURCE == "push" && $CI_PROJECT_ROOT_NAMESPACE == "project"
    # The last rule above blocks manual and scheduled pipelines on non-default branch. The rule below allows them:
    - if: $CI_PIPELINE_SOURCE == "schedule" && $CI_PROJECT_ROOT_NAMESPACE == "project"

to

    # Run on push to branch or scheduled branches.
    - if: $CI_COMMIT_BRANCH && $CI_PROJECT_ROOT_NAMESPACE == "project"

Alternatively if we do want CI to run on tags then maybe we should set COMPOSER_ROOT_VERSION in the rules section, where we can use different variables to construct it depending on the pipeline source? This is allowed according to docs: https://docs.gitlab.com/ee/ci/yaml/#rulesvariables - this also feels a bit more neat than relying on one or the other to be set in ${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}${CI_COMMIT_BRANCH}-dev".