nix: Use packages instead of buildInputs

In practice, `packages` sets the value for `nativeBuildInputs`. This has the following advantages:

- `buildInputs` is for runtime dependencies. `nativeBuildInputs` is for
  build dependencies (this usually not an issue unless cross compiling)
- `shellHook`s will actually run, allowing packages to safely append to
  variables like `XDG_DATA_DIRS`
- It's more idiomatic and how it's recommended to be used in the docs

flake.nix

@@ -146,7 +146,7 @@
       });
 
       devShells.default = pkgs.mkShell {
-        buildInputs = with pkgs; [
+        packages = with pkgs; [
           # NOTE (aseipp): explicitly add rust-src to the rustc compiler only in
           # devShell. this in turn causes a dependency on the rust compiler src,
           # which bloats the closure size by several GiB. but doing this here