flake: fix `lib.trivial.version` when used from a flake

A lot of fetchers from Nix's own `libfetchers` also provide the
information that `lib.trivial` aims to expose with
`version`/`versionSuffix`/`revision`. In fact you don't even need a
`nixpkgs` channel to get a proper version suffix because of that!

Unfortunately this isn't used currently. When using the
nixpkgs flake, but not `nixpkgs.lib.nixosSystem` to build a NixOS
configuration, the version will always be `YY.MMpre-git`. One example is
e.g. `colmena` which evaluates configurations via
`import (npkgs.path + "/nixos/lib/eval-config.nix")`.

This patch ensures that the version suffix (i.e. the normalized last
modified date + git revision) is correctly exposed in `lib.trivial`.
Additionally, the change is injected into the following locations:

* `lib`: with that, something like

      $ nix eval nixpkgs#lib.trivial.version
      23.05.20230921.cf8bf79

  is working fine (i.e. rather than `23.05pre-git`).

* `legacyPackages` to make sure that e.g. `legacyPackages.<system>.nixos`
  has correct version info. This also applies to everything else using
  `pkgs.lib.trivial` for that purpose.

* `overlays.default` which can be applied to a `nixpkgs` and changes the
  previous `pkgs.lib` from said `nixpkgs` to also contain the correct
  `version`/`revision`/etc..

  This is useful for people using `nixpkgs` as flake input, but
  importing it manually with

      import inputs.nixpkgs { }

Co-authored-by: Linus Heckemann <git@sphalerite.org>

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