docs: Update documentation on metal support via NODE_LLAMA_CPP_METAL env

[timothycarambat]

Description of change

Update the documentation of enabling or disabling metal support via the NODE_LLAMA_CPP_METAL environment variable. The current example only shows how to disable metal support when building llama.cpp bindings via the CLI command.

Pull-Request Checklist

• [x] Code is up-to-date with the master branch
• [x] npm run format to apply eslint formatting
• [x] npm run test passes with this change
• [ ] This pull request links relevant issues as Fixes #0000 N/A
• [ ] There are new or updated unit tests validating the change N/A
• [x] Documentation has been updated to reflect this change
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[giladgd]

@timothycarambat This environment variable only affects the compilation of llama.cpp; setting it in runtime like the example you provided is not supported. There's already documentation for this environment variable here.

Thanks for the contribution anyway :)

[giladgd]

@timothycarambat node-llama-cpp builds llama.cpp at runtime only if no binaries are available. It looks to you that this code works because you run it locally on a dev environment that doesn't have any pre-built binaries. It won't work on a second run or on the module that's uploaded to npm