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
By moving the tests subdirectory to t, we gain the ability to run `yath
test` with no arguments from inside `nix develop` in the root of the
the repo.
(`nix develop` is necessary in order to set the proper env vars for
`yath` to find our test libraries.)
