Allow env variable to override metal resource path

[codesoda]

Implementation for #1397

If an ENV variable called WHISPER_METAL_PATH_RESOURCES is available, then the ggml-metal.metal file is loaded from that path instead of using the bundle#pathForResource method.

---

Testing

🟥 Run main and expect a load (null) error.

Use make to compile the main binary and move it to a folder outside the project root.

$ ./main -m models/ggml-base.bin test.wav
...
ggml_metal_init: allocating
ggml_metal_init: found device: Apple M1
ggml_metal_init: picking default device: Apple M1
ggml_metal_init: loading '(null)'
ggml_metal_init: error: Error Domain=NSCocoaErrorDomain Code=258 "The file name is invalid."
whisper_init_state: ggml_metal_init() failed
error: failed to initialize whisper context

🟩 Launch again, passing the project root path, where the ggml-metal.metal file exists. Success!

$ WHISPER_METAL_PATH_RESOURCES=/Users/codesoda/projects/whisper.cpp ./main  -m models/ggml-base.bin test.wav
...
ggml_metal_init: allocating
ggml_metal_init: found device: Apple M1
ggml_metal_init: picking default device: Apple M1
ggml_metal_init: loading '/Users/codesoda/projects/whisper.cpp/ggml-metal.metal'
ggml_metal_init: loaded kernel_add                            0x155707730 | th_max = 1024 | th_width =   32
...

[ggerganov]

Thanks for fixing this. I'm bringing a large update in #1422
Would you mind rebasing and adjusting the PR to that branch to reduce conflicts?

[codesoda]

Hope that update helps @ggerganov