[$100 bounty] Compiling openpilot on macOS

[maxime-desroches]

We recently brought back some support for installing most dependencies on macOS (see https://github.com/commaai/openpilot/pull/32819). The current test just invokes scons -h in order to verify that we have the correct dependencies but it does not actually build anything. We want to continue with the next step: compiling openpilot.

The goal:

• compile openpilot on an arm64 macOS machine. (You don't need an arm64 macOS machine for doing this bounty since everything will be validated by our CI runner.)

How to do it:

• modify the current build_mac test such that it fully invokes scons instead of just calling scons -h
• make the test pass