nix-flake: Add nix-workspace subflake

The subflake nix-workspace provides a derivation containing a fully fetched
and initialized west workspace, located in the nix store (read-only). The flake
is generated by a python script, which reads the west.yml manifest and creates
corresponding inputs and derivations.

This output is also re-exported by the main flake.

Note that to re-generate the subflake, the script must be run in an active west
workspace. Once it is generated, an existing workspace is no longer necessary
to build its outputs.

For best results, run `nix fmt` after re-generating nix-workspace/flake.nix.

Signed-off-by: Sarah Renkhoff <sarah.renkhoff@navimatix.de>