Run singularity test only in merge-queue

[mashehu]

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).

Originally posted by @edmundmiller in #3098