Honor $NIX_STORE_DIR.

Dependencies

• [2] CQTN62OH Die tabs die
• [3] 3PNG7NIB Remove trailing whitespace
• [4] PRDDAI6W Minor tweak
• [5] XE6HD7BP Provide the nix-cache-info file required by the binary cache substituter
• [6] EWRCO2ND Give Hydra binary caches a low priority
• [*] J5UVLXOK * Start of a basic Catalyst web interface.

Change contents

• replacement in src/lib/Hydra/Controller/Root.pm at line 245
  [3.195]→[3.195:230](∅→∅)
  − "StoreDir: /nix/store\n" .
  [3.195]

  [2.2722]
  + "StoreDir: " . ($ENV{NIX_STORE_DIR} || "/nix/store") . "\n" .