Test pipelines with Singularity & Conda

[mirpedrol]

Description of feature

Pipelines are only tested with Docker. We should add tests with Singularity and Conda too.
Test with Singularity and Conda only on PRs to master + workflow dispatch. To avoid running the tests too often.
Linting: Singularity should be required, but Conda must be optional (opt-out) for pipelines which use custom Docker containers.

[edmundmiller]

Kinda a seperate issue but thought it might be a good place for the discussion.

I was thinking about this in terms of nf-core/modules.

Since we use the merge_queue, what if we only ran singularity and conda when the merge_queue is ran? It seems like the conda environment creation in some modules takes a really long time to get created(and in nf-test it creates the environment every time).