ref(home-manager): don't use home-manager when building the host

When rebuilding the host (through `nixos-rebuild switch --flake`) I
don't want to rebuild also my home-manager configuration. I want these
to be two different steps.

I rebuild the home-manager configuration more frequently and it's a
waste of time and CPU to rebuild the world every time.

This is a pretty large refactoring:
- move checks back into the flake: if I modify a check, the
  configuration for `pre-commits` is not regenerated, as the file with the
  checks is not monitored with `direnv` (I could probably configure it for
  it, but not now)
- remove `home.nix` from the host level configuration
- introduce a `mkHomeManagerConfiguration` function to manage the
  different user@host
- fix a warning with the rust overlay

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