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

t/s3-backup-test.pl

@@ -3,7 +3,6 @@ use warnings;
 use File::Basename;
 use Hydra::Model::DB;
 use Hydra::Helper::Nix;
-use Nix::Store;
 use Cwd;
 
 my $db = Hydra::Model::DB->new;