doc: build-script: use cfg(unix), not a hardcoded x86_64 triple
doc: build-script: use cfg(unix), not a hardcoded x86_64 triple

This used to not be possible, but it is now, so let's promote best practices.
tshepang/cargo,vojtechkral/cargo,tshepang/cargo,vojtechkral/cargo,AndiDog/cargo,rust-lang/cargo,durka/cargo,tsurai/cargo,tsurai/cargo,achanda/cargo,AndiDog/cargo,sbeckeriv/cargo,rust-lang/cargo,durka/cargo,nipunn1313/cargo,rustyhorde/cargo,AndiDog/cargo,eminence/cargo,eminence/cargo,AndiDog/cargo,tshepang/cargo,durka/cargo,tsurai/cargo,rustyhorde/cargo,achanda/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,sbeckeriv/cargo,rust-lang/cargo,nipunn1313/cargo,achanda/cargo,nipunn1313/cargo,vojtechkral/cargo,tsurai/cargo,rustyhorde/cargo,sbeckeriv/cargo,achanda/cargo,vojtechkral/cargo,AndiDog/cargo,tshepang/cargo,sbeckeriv/cargo,durka/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo