cmake: Fix cmake builds on macOS.

[etra0]

If we have a null path, we then do a last check in bin/ggml-metal because that's where the metal file is located in the case of cmake builds.

Fixes #1419.

[codesoda]

@etra0 Have you also had issues with #1397 / #1415 and being unable to find ggml-metal.metal on disk in release builds?

[ggerganov]

Yup, the proper fix would be #1415
Hardcoding "bin/..." is a hack on top of an existing hack. With the env variable it should be a bit more generic

[etra0]

I agree.