Add support for nodejs-mobile

[jaimecbernardo]

Hi,
nodejs-mobile is a fork of node used to run Node.js applications on Android and iOS.

It also allows building and running native modules. For this, it tries to use nodejs-mobile-gyp instead of node-gyp. The way it does this is setting the npm_config_node_gyp to point to nodejs-mobile-gyp. However, npm, since v7+, has started overwriting this environment variable before passing it to modules, such as node-pre-gyp.

This PR makes it so we check for the NODEJS_MOBILE_GYP environment variable to try to set node_gyp's location before trying the other methods. This variable is already used in some projects related to nodejs-mobile, so we decided to pick it:
https://github.com/nodejs-mobile/prebuild-for-nodejs-mobile/blob/44687f6cb4c0375e0f4ef063ce3321350f159241/bin.js#L367
https://github.com/nodejs-mobile/nodejs-mobile-react-native/blob/794f395152f25d1c5ccf9cc794eafa8ce58ad541/scripts/ios-build-native-modules.sh#L116
https://github.com/nodejs-mobile/nodejs-mobile-react-native/blob/794f395152f25d1c5ccf9cc794eafa8ce58ad541/android/build.gradle#L433
https://github.com/okhiroyuki/nodejs-mobile-cordova/blob/9d02c76700bba851241922d7632555546b4ee771/src/android/build.gradle#L243

The original motivation for this change was building sqlite3, which uses "@mapbox/node-pre-gyp", for nodejs-mobile. This is currently possible by adding this overrides field to a package.json of a project that's using sqlite3, but in theory it should help improve compatibility with other modules as well:

  "overrides": {
    "sqlite3": {
      "@mapbox/node-pre-gyp": "github:JaneaSystems/mapbox-node-pre-gyp#add-support-for-nodejs-mobile"
    }
  }

We'd consider this change would be safe for current uses, since in theory a process which sets an environment variable called NODEJS_MOBILE_GYP is likely to want to build for nodejs-mobile.

@staltz , FYI, since this information may help unblock building other modules in nodejs-mobile.