protobuf: fix protoc used when cross-compiling

[FliegendeWurst]

This basically inlines the fix applied in most consumers of protobuf.

OpenCV 4 derivation is simplified to automatically use the right protoc. (It didn't work before anyhow.)

Things done

• Built on platform(s)
  • x86_64-linux: pkgsCross.aarch64-multiplatform.protobuf_{21,23,24,25,26,27,28} pkgsCross.aarch64-multiplatform.opencv4 builds
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

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[GaetanLepage]

LGTM.
I would like another ACK from someone who is more familiar with cross-compilation.

[misuzu]

LGTM!

[FliegendeWurst]

So I thought about this again, and I think adding a setup-hook as below would be the better solution.
It has the benefit that the cross-compiled protobuf package can be used as dev package on the host, which would otherwise not be possible.
(Actually I am not sure that makes much sense with the way buildPlatform etc. work at the moment.)

ProtobufCMakeFlags() {
  cmakeFlagsArray+=(
    -DPROTOC_EXE="@build_protobuf@/bin/protoc"
  )
}

preConfigureHooks+=(ProtobufCMakeFlags)

^ has essentially the same effect as the previous patch to the CMake files, overriding the path to protoc.

[rnhmjoj]

Ok for the monero part.

[misuzu]

Tested by building:

• pkgs.monero-cli
• pkgs.monero-gui
• pkgs.opencv4
• pkgsCross.riscv64.opencv4
• pkgsCross.riscv64.protobuf_21
• pkgsCross.riscv64.protobuf_23
• pkgsCross.riscv64.protobuf_24
• pkgsCross.riscv64.protobuf_25
• pkgsCross.riscv64.protobuf_26
• pkgsCross.riscv64.protobuf_27
• pkgsCross.riscv64.protobuf_28
• pkgsCross.riscv64.protobuf_29

[GaetanLepage]

Thanks guys !