nix: Fix --pr-only

Without this change, running "superflore-gen-nix --pr-only" fails with
this error:

    !!!! Failed to file PR!
    !!!! reason: Cmd('/nix/store/cjw8ydwnifad0wdlkhpsyrdm18bcmg63-git-minimal-2.47.2/bin/git') failed due to: exit code(128)
      cmdline: /nix/store/cjw8ydwnifad0wdlkhpsyrdm18bcmg63-git-minimal-2.47.2/bin/git clone -v --branch=master -- https://github.com/./False False
      stderr: 'Cloning into 'False'...
    remote: Not Found
    fatal: repository 'https://github.com/./False/' not found
    '

Signed-off-by: Michal Sojka <michal.sojka@cvut.cz>

Author: wentasah

superflore/generators/nix/run.py

@@ -59,7 +59,7 @@ def main():
         if not args.output_repository_path:
             parser.error('Invalid args! no repository specified')
         try:
-            prev_overlay = RepoInstance(args.output_repository_path, False)
+            prev_overlay = NixRosOverlay(args.output_repository_path, False)
             msg, title = load_pr()
             prev_overlay.pull_request(msg, title=title)
             clean_up()