There is no build_proton.sh file in Proton nowadays:
https://github.com/ValveSoftware/Proton
But this file was there before in Proton 3.7:
https://github.com/ValveSoftware/Proton/tree/proton_3.7
I think you are trying to use some outdated guide. You can either use build instructions from this repo or use the old branch with your guide. But I think Proton is not supposed to be built under macOS.
proton 3.7x1 2020-05
macOS 10.11.6, installed all the requirements using
brew install wget mingw-w64 cmake nasm libtool automake meson glslang, cloned the repo, updated it usinggit submodule update --init, and the moment of truth, let's run./build_proton.sh!..-bash: ./build_proton.sh: No such file or directoryWhat do I do?