Support shorthand nixpkgs references

[JamieMagee]

What would you like Renovate to be able to do?

In the nix manager we are currently looking for github:nixos/nixpkgs/nixos-22.05, however the shorthand nixpkgs/nixos-22.05 is also officially supported.

There is a full list of supported shorthands, however we only need to worry about nixpkgs for now.

If you have any ideas on how this should be implemented, please tell us here.

Update the extraction regex, or add another regex.

Is this a feature you are interested in implementing yourself?

Yes