use local clone to limit bandwidth usage

Dependencies

• [2] O25D52TA initial support for mercurial
• [3] JTRG7RDQ add support for git as jobinput
• [*] OOQ2D3KC * Refactoring: move fetchInput out of hydra_scheduler into a separate

Change contents

• replacement in src/lib/Hydra/Helper/AddBuilds.pm at line 410
  [2.1878]→[2.1878:1929](∅→∅)
  − ("nix-prefetch-hg", $uri, $revision));
  [2.1878]

  [2.1929]
  + ("nix-prefetch-hg", $clonePath, $revision));