Follow OS conventions for storing cache

Hi, currently, `.electron-gyp` is stored in the `home` directory. I think it would be much nicer if the default could be changed so that it is stored in the expected cache directory for the OS as `node-gyp` did [here](https://github.com/nodejs/node-gyp/commit/8a83972743).

I skimmed through the codebase and I think the only thing that would have to be done is to change [this line](https://github.com/electron/rebuild/blob/07c426138e356ad129bf6c758d38b451c945c78d/src/constants.ts#L4) to use [env-paths](https://www.npmjs.com/package/env-paths) to get the system cache directory (as `node-gyp` did in the referenced commit) and add it to the package.json.

I'm not familiar with typescript or this codebase, so I may be missing something. If I have time I might create a pull request for this.

Start with src/constants.ts at the linked line and inspect package.json for dependency declarations. Compare the referenced node-gyp change and env-paths usage, then verify that .electron-gyp resolves to the expected OS cache directory and that the package still builds and tests successfully.

Written by the indexing model from the issue text.