As discussed in #3430 it would be desirable to have the option to use the centralized configs also on offline HPCs, if they are available as a local copy. This PR implements this change as suggested by @bentsherman:

Rationale:

• The new config parser does not allow for try, if or def on a top-level, so the old try notation needed updating.
• Normally, we include the nf-core custom config from GitHub, the custom_config_base defaults to the nf-core/configs repository.
• If an execution environment does not have access to the internet, the pipeline run would, however, fail because the custom config is not available.
• Therefore, we do not want to include the custom config from GitHub if the NXF_OFFLINE environment variable is set, yet allow for a local path.
• The institutional configs should be included even in offline mode, if they are locally available.

Additionally, it fixes the downstream issues that arose by this change:

• nf-core pipelines lint had to be significantly changed to accommodate to the fact that we now have multiple old versions of that code.
• nf_core/pipeline-template/.github/workflows/config_tests.yml is a new GitHub Action to test, if a pipeline actually has the institutional configs available. It is run on pull requests to main, such that any issues should be caught prior to a pipeline release.
• nf-core pipelines create was adapted so that the aforementioned GitHub Action is not included in a customized template when opting out of the centralized nf-core profiles.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated