ffmpeg module does not rebuild according to `arch`

We are using `ffmpeg-static` in our Electron app, but when the universal build is created, only the arm64 ffmpeg binary exists. I have confirmed that the `ffmpeg-static` module can be rebuilt with an arch param (`npm rebuild --arch=arm64 -f ffmpeg-static`, then confirmed with `file node_modules/ffmpeg-static/ffmpeg`) and the correct binary is created/downloaded. But when running the universal build, only os.arch() seems to be respected (which is an M1 machine in CircleCI).

If there were hooks in the universal build process, or some env vars set, I could manually rebuild this one module according to an arch. For example this ffmpeg module does read `process.env.npm_config_arch`, so if I were able to set that in between the x64 and arm64 builds (or `electron/universal` did), the module would build the correct ffmpeg binary.