fix: symbolic linking (ln -s) with pre-existing installation

[dlaehnemann]

Description

This is an issue we came across very far downstream, when using the setup-miniconda GitHub Action in multiple steps of the same GitHub Actions job implicitly (through the snakemake GitHub Action that uses it, which we use for two subsequent steps).

Basically, the during first use of the snakemake GitHub Action, setup-miniconda installs miniforge without any issues. During the second install, it throws this error:

Running installer...
  /usr/bin/bash /home/runner/work/_temp/e83768cc-2602-4a3d-a62b-6a1cc1af9dd4.sh -f -b -p /home/runner/miniconda3
  PREFIX=/home/runner/miniconda3
  Unpacking bootstrapper...
  Warning: ln: failed to create symbolic link '/home/runner/miniconda3/_conda': File exists
  
  ln: failed to create symbolic link '/home/runner/miniconda3/_conda': File exists
Error: The process '/usr/bin/bash' failed with exit code 1

Finding out where that linking actually happens, so where the installers actually take their script code from, was a bit of treasure hunt. But here we are. The code I am suggesting to modify was only recently added in #1090 and thus seems very likely to be the cause of this. But if this forcing of the symlinking does not look like the correct fix to you, I'd be glad to stand corrected.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[dlaehnemann]

I'm not sure if it makes sense to add a test case for this and whether this would need to be here, or rather somewhere downstream (e.g. miniforge?). It would basically mean running an installer twice, to ensure the installer works, even if a version of conda already exists...

[dlaehnemann]

Pinging @marcoesters , as he reviewed the pull request #1090 that brought in this code.

[marcoesters]

I'm not sure if it makes sense to add a test case for this and whether this would need to be here, or rather somewhere downstream (e.g. miniforge?). It would basically mean running an installer twice, to ensure the installer works, even if a version of conda already exists...

I agree that this is niche, especially since the -f flag rather brittle. I'm okay with not having a specific test here.

[marcoesters]

Thank you!

[dlaehnemann]

Perfect, that was quick. Many thanks!