Find a path that works for just on macos

[ghickman]

While we currently have no CI using the macos target, it would be useful to have at least pipeline using it since both that is used there by opensafely-cli (which should also target it). Currently this action places just in a location isn't liked by CI when called from there.

I think we have two choices:

• set up a conditional for macos to change the output location
• find a location that works for all OSen

I suspect there's an ideal path for each OS so it's probably easier from a future-us-debugging standpoint to do a conditional.

[bloodearnest]

More importantly than final executable path, we need to download binaries compiled for windows/macos, not linux