Motivation for this change

See #44831, though this is a more extreme approach.

Not sure yet if this is a stupid idea or a good way to quickly apply Mesa updates. This mainly makes sense under the assumption that minor/patch releases only update the drivers (mesa.drivers) anyway. The plan would be to freeze mesa at X.Y.1 (first stable release of a series) and update mesa_drivers to X.Y.1+N without going through staging first. Or another idea would be to simultaneously update mesa via staging and mesa_drivers via master.

Personally I'm already using that approach to test Mesa updates and it seemed to be fine so far (though I only did very minimal testing). However with lsof I can always see a few references to the previous Mesa version, though mainly(/only?) for libgbm IIRC (e.g. /nix/store/***-mesa-X.Y.Z/lib/libgbm.so.1.0.0). For that reasons it might make sense to simultaneously update mesa via staging or implement the approach from #44831.

Note: The update to 20.1.10 doesn't make sense since 20.2.1 is already in staging, it only serves as an example.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.