trivial: make symlinkJoin support pname+version alone (#344645)

## `symlinkJoin` {#trivial-builder-symlinkJoin}

This can be used to put many derivations into the same directory structure. It works by creating a new derivation and adding symlinks to each of the paths listed. It expects two arguments, `name`, and `paths`. `name` is the name used in the Nix store path for the created derivation. `paths` is a list of paths that will be symlinked. These paths can be to Nix store derivations or any other subdirectory contained within.

This can be used to put many derivations into the same directory structure. It works by creating a new derivation and adding symlinks to each of the paths listed. It expects two arguments, `name`, and `paths`. `name` (or alternatively `pname` and `version`) is the name used in the Nix store path for the created derivation. `paths` is a list of paths that will be symlinked. These paths can be to Nix store derivations or any other subdirectory contained within.

Here is an example:

```nix

# adds symlinks of hello and stack to current build and prints "links added"

    as a easy way to build multiple derivations at once.

   */

  symlinkJoin =

    args_@{ name

    args_@{

      name ?

        assert lib.assertMsg (args_ ? pname && args_ ? version)

          "symlinkJoin requires either a `name` OR `pname` and `version`";

        "${args_.pname}-${args_.version}"

    , paths

    , preferLocalBuild ? true

    , allowSubstitutes ? false

  disableUpdateNotifier ? true,

}:

symlinkJoin rec {

symlinkJoin {

  pname = "turbo";

  inherit (turbo-unwrapped) version;

  name = "${pname}-${version}";

  nativeBuildInputs = [ makeBinaryWrapper ];