Switch to new Nix bindings, update Nix for that

Implements support for Nix's new Perl bindings[1]. The current state
basically does `openStore()`, but always uses `auto` and doesn't support
stores at other URIs.

Even though the stores are cached inside the Perl implementation, I
decided to instantiate those once in the Nix helper module. That way
store openings aren't cluttered across the entire codebase. Also, there
are two stores used later on - MACHINE_LOCAL_STORE for `auto`,
BINARY_CACHE_STORE for the one from `store_uri` in `hydra.conf` - and
using consistent names should make the intent clearer then.

This doesn't contain any behavioral changes, i.e. the build product
availability issue from #1352 isn't fixed. This patch only contains the
migration to the new API.

[1] https://github.com/NixOS/nix/pull/9863

src/lib/Hydra/Controller/Root.pm

@@ -395,7 +395,7 @@ sub narinfo :Path :Args(StrMatch[NARINFO_REGEX]) {
         my ($hash) = $narinfo =~ NARINFO_REGEX;
 
         die("Hash length was not 32") if length($hash) != 32;
-        my $path = queryPathFromHashPart($hash);
+        my $path = $MACHINE_LOCAL_STORE->queryPathFromHashPart($hash);
 
         if (!$path) {
             $c->response->status(404);