Actions CI - use include for old Nextflow version

[ewels]

All @nf-core pipelines must test two versions of Nextflow - the minimum required by the pipeline and the latest release.

However, it's quite common for pipelines to test multiple config profiles as well. With the normal job matrix setup this means that all test profiles are run twice, for both versions of Nextflow. This is probably overkill.

A nicer method is to use the include keyword to inject the old Nextflow version run for just the test profile as I did here: https://github.com/nf-core/diaproteomics/pull/131/files

If we like this, need to update the linting tests to allow this. Could even also update the template and make the lint tests force this style.