New feature as of electron-chromedriver >= 9.0.0
and electron-mksnapshot >= 9.0.2:
an environment variable "ELECTRON_CUSTOM_VERSION",
which allows downloading the specified (Electron-vendored)
version of chromedriver and mksnapshot,
irrespective of the versions of electron-chromedriver
or electron-mksnapshot (node modules) used to download them.
We can use the latest electron-chromedriver and electron-mksnapshot
now, if we want. Just set ELECTRON_CUSTOM_VERSION to the right version
(handled automatically based on "electronVersion" in package.json).
