Support `npm_config_platform_arch` as alternate to `npm_config_arch`

`npm install --platform-arch=arm64` will set `npm_config_platform_arch=arm64` which is respected by `node-gyp` and `install-artifact-from-github`.

So `prebuild-install` should respect this too. Currently i need to set `npm_config_arch` and `npm_config_platform_arch`.