Fix registerRoot

Fixes #175.

src/lib/Hydra/Helper/Nix.pm

@@ -79,6 +79,7 @@ sub gcRootFor {
 sub registerRoot {
     my ($path) = @_;
     my $link = gcRootFor $path;
+    return if -e $link;
     open ROOT, ">$link" or die "cannot create GC root `$link' to `$path'";
     close ROOT;
 }