Allow Perl bindings to perform operations on arbitrary stores

[Ma27]

Describe the bug

Store operations done via Nix's Perl bindings are always made on the "default" store (i.e. auto) because open() doesn't accept a store uri and caches the store internally:

https://github.com/NixOS/nix/blob/2.19.3/perl/lib/Nix/Store.xs#L23-L35

Steps To Reproduce

n/a

Expected behavior

Being able to perform operations on arbitrary stores.

nix-env --version output

nix-env (Nix) 2.19.3

Additional context

I discovered this by debugging Hydra: I wondered why a lot of builds were claiming to be garbage collected even though this wasn't the case:

The cause for that was that my Hydra doesn't run builds on localhost, but exclusively on remote machines it connects to via ssh.
Then, the store paths are downloaded into the store specified via store_uri in hydra.conf.
Because I use a flat-file binary cache (file:///var/lib/hydra/store), sub isLocalStore in Hydra returns true and thus the output paths of a Hydra build are checked with isValidPath from the Perl bindings:

https://github.com/NixOS/hydra/blob/7f5889559e3118c66763428c410958dcec6a6fa8/src/lib/Hydra/Controller/Build.pm#L80-L86

This however doesn't use the binary cache for that, but the machine's /nix/store and thus wrongly claims that the build product was garbage collected.

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