nf-test Github actions decompress automatically tar.gz outputs when they should not

[toniher]

Have you checked the docs?

• nf-core website: troubleshooting
• nf-core modules documentation

Description of the bug

It seems that tar.gz outputs are decompressed automatically in Github actions (singularity, docker, conda) and it is then not possible to test if a file output is a compressed file.
Details here: #5510

No problem on Gitpod instance.

Command used and terminal output

Github Action

Relevant files

.test files

System information

Github action

[toniher]

For the sake of information, there seems to be a similar bug in Conda: conda/conda#3798

[toniher]

More understandable reference: #5510 (comment)