[nix] allow buildIdris output to be used as a dependency in other buildIdris calls more directly

[mattpolzin]

Description

Following in the footsteps of a recent change to the Nixpkgs variant of buildIdris, this PR just makes it a bit easier to work with the output of the buildIdris function as an input to another Idris package. The changes are backwards compatible so they just provide more flexibility and better ergonomics going forward.

Before:

let myLib = buildIdris { ... };
in buildIdris {
  ...
  idrisLibraries = [ (myLib.library {}) ];
}

After:

let myLib = buildIdris { ... };
in buildIdris {
  ...
  idrisLibraries = [ myLib ];
}

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).