If users manually run `npm install`, we want to
make sure the correct Electron-vendored chromedriver and mksnapshot
are downloaded. (Requires ELECTRON_CUSTOM_VERSION to be set properly.)
This postinstall script sets that var and gets the right binaries,
even if the ELECTRON_CUSTOM_VERSION env var wasn't manually set.
(The bootstrap script already handles this for bootstrapping,
but not for manually running "npm install" in the scripts dir.)