nixos/man-db: set preferLocalBuild = true

When we _must_ build, prefer building locally since we probably already have the manpages here. Otherwise, Nix would send all the manpages to a remote builder which isn't likely to be much faster since building the DB is single-threaded. Hopefully a best-of-both worlds between keeping or removing 39e63f110e.

nixos/modules/misc/man-db.nix

@@ -82,6 +82,7 @@ in
           pkgs.runCommand "man-cache"
             {
               nativeBuildInputs = [ buildPackage ];
+              preferLocalBuild = true;
             }
             ''
               echo "MANDB_MAP ${cfg.manualPages}/share/man $out" > man.conf