fetchFromSourcehut: expose tag in derivation attrs

Other fetchers (fetchFromGitHub, fetchFromGitLab, etc.) expose tag such
that the attribute src.tag can be accessed. Add this to
fetchFromSourcehut as well.

I need this for a script I'm working on.

Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>

pkgs/build-support/fetchsourcehut/default.nix

@@ -90,6 +90,7 @@ makeOverridable (
   in
   cases.${fetcher}.fetch cases.${fetcher}.arguments
   // {
+    inherit tag;
     rev = rev';
     meta.homepage = "${baseUrl}";
   }