fix(nix): Actually use nixd LSP

[ItsDrike]

📑 Description

In 44e8c95 an isssue with trying to install nixd via mason was fixed, by removing nixd from required mason packages and instead adding it as a required dependency for the pack, that should be present in the PATH.

However, normally, mason would've also registered nixd to lspconfig, but since mason isn't being used, nixd LSP was never registered, making this requirement pointless.

This actually registers nixd languge server through astrolsp plugin.

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).