This PR fixes the warning from entering the nix development shell:
```
evaluation warning: 'hostPlatform' has been renamed to/replaced by 'stdenv.hostPlatform'
```

Decided to go with `zed-editor = mkZed pkgs;` instead of `zed-editor =
packages.${pkgs.stdenv.hostPlatform.system}.default;`, because it is
simpler and with my understanding it is logically equivalent (i.e. we
are getting `packages.<system>.default` which we can see in the
definition of packages is equal to `mkZed pkgs;`).

Release Notes:

- N/A