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/hydra-evaluator/hydra-evaluator.cc

@@ -38,7 +38,7 @@ class JobsetId {
     friend bool operator!= (const JobsetId & lhs, const JobsetName & rhs);
 
     std::string display() const {
-        return str(format("%1%:%2% (jobset#%3%)") % project % jobset % id);
+        return boost::str(boost::format("%1%:%2% (jobset#%3%)") % project % jobset % id);
     }
 };
 bool operator==(const JobsetId & lhs, const JobsetId & rhs)