Add a flake output for the helix cogs

[MatrixManAtYrService]

You can already ask the flake to build helix for you, this PR adds the ability to build the helix cogs also:

❯ nix build .#helix-cogs
❯ ls result/cogs/helix
╭───┬─────────────────────────────────────┬──────┬─────────╮
│ # │                name                 │ type │  size   │
├───┼─────────────────────────────────────┼──────┼─────────┤
│ 0 │ result/cogs/helix/commands.scm      │ file │ 14.8 KB │
│ 1 │ result/cogs/helix/misc.scm          │ file │  1.8 KB │
│ 2 │ result/cogs/helix/static.scm        │ file │ 42.5 KB │
│ 3 │ result/cogs/helix/configuration.scm │ file │  6.5 KB │
│ 4 │ result/cogs/helix/editor.scm        │ file │  1.8 KB │
╰───┴─────────────────────────────────────┴──────┴─────────╯

By default, cargo xtask code-gen modifies ~/.config. I find this easy to lose track of. For instance, I had plugins working on one machine and not on the other and I was scratching my head about what was different between those machines.

This change makes it easy to just get the cogs without modifying my config, which is handy for creating dev environments which are isolated from the rest of my system (like this one, which is a work in progress).

[Talia-12]

I'm using this now in my config with no issues