Don't trigger toRustTarget deprecation warning

[SuperSandro2000]

Merging attrs with // triggers lib.warn. I try to work around this by removing the attr and then adding it back, so it is not being evaluated.

[nyabinary]

Won't get very far considering their no active maintainer rn :P https://github.com/nix-community/fenix/issues/195

[winterqt]

Please fix eval -- how did you test this?

[eureka-cpu]

@SuperSandro2000 can you follow up here? It would seem this is still a work in progress, so I will mark it as a draft for now.