Update Nim 2 on CI

[AlejandroCabeza]

Description

In Github Actions, Nim's runtime for version 2 has been fixed to a specific commit. This is due to a newer bersion breaking it.
The problem is in two fronts:

1. SAT solver has been made the default and, while that can be worked around, there's a dependency resolution issue with it. Legacy is able to solve deps while SAT isn't. This is reported in this Nimble Issue).
2. Even if the workaround is applied, there are a lot of tests that begin failing. This hasn't been deeply investigated yet.

That being said, both these issues begin happening when using this commit (cc4c9251f0fbf5e2c93e6815db402a0e27e1110d) on version-2-0 from Nim's repo.

To work around those two issues we created a PR that pins the Nim's ref on Github Actions to the commit just before the problematic one.

Solution

Periodically check when Nim's version-2-0 branch is stable again (fixes problem 2., at least). When that is fixed, then:

1. Revert to using version-2-0 instead of the commit hash.
2. 1. Update the required jobs to merge a PR:

        linux-amd64 (Nim version-2-0)   
        macos-amd64 (Nim version-2-0)   
        windows-amd64 (Nim version-2-0) 
      

   2. Ideally, instead of selecting N jobs that are required, we would select the full workflow (ci.yml in this case) as required, so internal naming changes don't affect this setting.

Note

If need be, update the supported Nim version in the README accordingly.