Switch CI to nf-core/setup-nextflow action

[MillironX]

Switches the workflows in this repo and the template to point to use setup-nextflow, rather than try to manually install Nextflow in a run: block. This allows for simplifying the execution matrix, and removes the need to keep track of whether a stable or edge release is actually the most recent.

PR checklist

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

[ewels]

Nice!

Can maybe also remove the CAPSULE_LOG: none env vars too (see nf-core/setup-nextflow#1 )

Also update if we're moving to nf-core, as discussed in Slack 👍🏻

Otherwise LGTM! 💥

[MillironX]

Converting to draft until two things are fixed:

1. Linting checks need to target the new matrix format
2. Action should point to https://github.com/nf-core/setup-nextflow

[MillironX]

Tests are now passing, so this is actually ready for review.

[codecov]

Codecov Report

Merging #1650 (0bab55b) into dev (7f8fa83) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1650   +/-   ##
=======================================
  Coverage   64.84%   64.84%           
=======================================
  Files          54       54           
  Lines        6270     6270           
=======================================
  Hits         4066     4066           
  Misses       2204     2204           

Impacted Files	Coverage Δ
nf_core/bump_version.py	89.83% <ø> (ø)
nf_core/lint/actions_ci.py	89.09% <100.00%> (ø)

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[ewels]

Just a few lurking CAPSULE_LOG env vars left I think? Otherwise the code LGTM 👍🏻