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

flake.lock

@@ -42,11 +42,11 @@
         "nixpkgs-regression": "nixpkgs-regression"
       },
       "locked": {
-        "lastModified": 1706629374,
-        "narHash": "sha256-KyAiLGxJ39fSY0cuq8EWAZQ4vaDdqAItSRP4+vjYvq8=",
+        "lastModified": 1707750141,
+        "narHash": "sha256-9qSzGQs/Rjf2i3UQjyaZUznZzYDHkLYro/1FTT1easg=",
         "owner": "NixOS",
         "repo": "nix",
-        "rev": "75ebb90a70f6320c1c7a1fca87a0a8adb0716143",
+        "rev": "c4ebb82da4eade975e874da600dc50e9dec610cb",
         "type": "github"
       },
       "original": {