devel/electron38: Update to 38.7.2

While here:

• switch to use USE_ELECTRON features of electron.mk for specifying node package manager, pre-fetching necessary node modules, and extracting the node modules into appropriate directories
• use USES=display for starting/stopping display server on testing
• specify TEST_ENV instead of listing env vars in test target

Changelog: https://github.com/electron/electron/releases/tag/v38.7.2