Allow --build_wasm on a mac system (#16761)
### Description
Changes allow downloading prebuilt protoc compiler when building
WebAssebly version on mac systems.
Otherwise it tries to build a js/wasm version of protoc and throws an
error while executing it: "protoc.js permission denied"
### Motivation and Context
I need to switch between my main working computer and a PC to make
changes to WebAssebly build. Would like not to do that anymore.
