Update setup-nim-action to v2

[dlesnoff]

I recently merged the update to setup-nim-action which contains some breaking changes (and breaks CI).
There might be some additional fixes to bring to our jobs.

The following recommended changes are described in the README of setup-nim-action:

1. Change cache key to clear cache if you are using it. The key can be anything if the cache will be cleared. Nothing to do if you are not using actions/cache
2. Remove caching choosenim if you are using it. setup-nim-action does not use choosenim now. Nothing to do if you are not caching choosenim
3. Remove yes and no-color parameters if you are using it. These parameters are not used now. Nothing to do if you are not using these parameters

This does not seem to impact our actual jobs, but have to be kept in mind when writing potential new jobs.

Currently the CI fails with an error message concerning an attempt to tar an archive that doesn't exist.

Run jiro4989/setup-nim-action@v2
Run "/home/runner/work/_actions/jiro4989/setup-nim-action/v2/install_nim.sh" \
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100   109  100   109    0     0   1037      0 --:--:-- --:--:-- --:--:--  1038
jq: error (at <stdin>:5): Cannot index string with string "ref"
Mon Jul  8 05:02:48 UTC 2024 [INFO] install nim 
tar: This does not look like a tar archive
xz: (stdin): File format not recognized
tar: Child returned status 1
tar: Error is not recoverable: exiting now
Error: Process completed with exit code 2.

Related: jiro4989/setup-nim-action#483
It seems that the new release introduced a bug with the 'stable' version.