buildRustCrate: do not overlay OUT_PATH on src/

[andir]

Motivation for this change

build.rs files might create files during the configurePhaes. Those files are supposed to go into
OUT_DIR (envirionment variable) and not be overlayed onto the source
tree. So far we did both. With this change we will only use the OUT_PATH.

fixes #78412

cc @kolloch

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[andir]

@GrahamcOfBorg build buildRustCrateTests

[kolloch]

This should also fix #74071. Just hoping that nothing depends on the old, odd behavior. Good luck! :)

[andir]

This should also fix #74071. Just hoping that nothing depends on the old, odd behavior. Good luck! :)

One of my short-term goals is going to be a topN rust crates build against latest crate2nix & buildRustCrate to have a bit more visibility on the impact of such changes. I compiled a few projects with this change and they all didn't fail. I am very confident (right now) that this change will not break more then what was broken before.