Build: enable nix-command on nix path-info call, silence stdout as …

…well

`nix-store -q` (and `--outputs`) doesn't seem to complain if a path
doesn't exist in a store anymore, so let's do it that way.

This approach works fine on Nix 2.3 as well, it only warns about
`experimental-features` being an unknown option, but that's silenced
along with the rest of stderr.

(cherry picked from commit 15b7d91)

src/lib/Hydra/Controller/Build.pm

@@ -79,7 +79,8 @@ sub isValidPathNixCLI {
     my $pid = fork();
     if (!$pid) {
         open STDERR, ">", "/dev/null";
-        exec "nix", "path-info", $path, "--store", getStoreUri();
+        open STDOUT, ">", "/dev/null";
+        exec "nix", "path-info", $path, "--store", getStoreUri(), "--option", "experimental-features", "nix-command";
     }
     waitpid $pid, 0;
     return $? == 0;