gl-testing: enable multi-arch protoc download for Apple Silicon

[YusukeShimizu]

Enable multi-architecture support to ensure proper protoc downloads based on the build environment.
This fixes issues on Apple Silicon (ARM64) where the x86_64 binary was previously being downloaded,
leading to the missing ld-linux-x86-64.so.2 error.

• Use the same protocol buffer compiler version across architectures
• Ensure Docker build success on Apple Silicon

[cdecker]

Very good contribution. I am surprised that the arch is passed through to the docker emulation directly, and not using qemu with x86 or x86_64. Today I learned :-)

If you undraft I will merge this asap ^^

[YusukeShimizu]

Thank you for confirming. I realized that this change was not enough to build on arm64.
To support building on ARM64, not only protoc but also all dependent binaries—such as bitcoin-core, cfssl, and cln—must be similarly addressed. In particular, cln (clm-vm) does not currently provide an ARM64 binary, raising concerns about increased support and maintenance costs.

Abandoning this approach and instead updating the make command to force emulation by specifying --platform linux/amd64 at build time is a simpler solution. However, running an Intel-based container on an Arm-based machine remains a best-effort measure, with associated risks of performance degradation and potential bugs.

I would appreciate your feedback.

[cdecker]

Indeed, we have been developing exclusively on amd64, and adding other archs, distros, and OSs may be a lot of surface. Parametrizing the binaries being downloaded is an excellent first step, and may be sufficient. Let's see if it works out :-)

[YusukeShimizu]

I removed the draft status from this PR.