pre-build-hook receives a invalid (non-existing) drvPath when remotely building with Op::BuildDerivation

[layus]

Describe the bug

When building with Op::BuildDerivation, the derivation is never written to the store. But the drvPath from the client is still available. It gets passed to pre-build-hook as first argument. So the hook ends up with a path to a nonexistent file, and no way to get information about the derivation.

Steps To Reproduce

Expected behavior

I would expect the content of the .drv file to be always available to the pre-build-hook. In this case writing the .drv to a temporary location, or to the store. I think it is not a problem to write it to the store.

Alternatively, if this is deemed unsafe, document the issue in pre-build-hook.

nix-env --version output

Additional context

NixOS/nixpkgs#256230 (comment)

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