nf-core download: Create container image symlinks so images are still found, even if registry config diverges.

[MatthiasZepper]

This addition to nf-core download addresses #2751, #2644, demultiplex#164 and multiple Slack issues: 1,2,3.

Dropping the hardcoded registry from the modules in favour of the flexible config parameters apptainer.registry, docker.registry, podman.registry and singularity.registry was initiated by Seqera, but so far Nextflow has not received concordant updates to also resolve to images downloaded from alternative registries as a fallback.

The code in this PR creates a bunch of symlinks to the downloaded images to ensure that they are found nonetheless. To ensure that it is possible to run the pipelines without symlinks by setting docker.registry or apptainer.registry to empty strings at runtime, the original registry is not preserved any more when downloading. This harbours a small risk of naming collisions, however, due to the symlinks it is anyway not possible to use the same $NXF_CACHE_DIR for pipelines using different containers with identical image names pulled from different registries.

Additionally, this PR modifies the pytest.yml Github Action such that the new tests are run at all, because they require Singularity.

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

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 75.00%. Comparing base (f64d486) to head (e64f1a7).
Report is 6 commits behind head on dev.

Files	Patch %	Lines
nf_core/download.py	90.47%	6 Missing ⚠️

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[Aratz]

Neat! Just had a couple minor questions for you