nixpkgs.lib: Include .version

[bb010g]

Used by release, version, and isInOldestRelease.

I know #1 already was refused, but I think this is valid for versioning lib itself. Example use: https://github.com/hercules-ci/flake-parts/blob/70d1b51a85170b46b57a2ad62308501651daa5c1/lib.nix#L214-L229

  # A best effort, lenient estimate. Please use a recent nixpkgs lib if you
  # override it at all.
  minVersion = "22.05";


in


if builtins.compareVersions lib.version minVersion < 0
then
  abort ''
    The nixpkgs-lib dependency of flake-parts was overridden but is too old.
    The minimum supported version of nixpkgs-lib is ${minVersion},
    but the actual version is ${lib.version}${revInfo}.
  ''
else

  flake-parts-lib

[roberth]

I'd prefer this to be handled upstream in nixpkgs itself, so that pulling the lib subtree stays simple and eventually Nix could support it natively.