dev-util/electron: Build node executable, fix automagic cups dependency.

dev-util/electron: Build node executable, fix automagic cups dependency.

Electron and apps based on it cannot rely on system NodeJS
for compatibility reasons. Fix this by building and using
a nodejs executable based on bundled nodejs code.

Additionally, fix automagic dependency on libcups and remove dependency on
system nodejs

Gentoo-Bug: https://bugs.gentoo.org/585116
Gentoo-Bug: https://bugs.gentoo.org/585490
Gentoo-Bug: https://bugs.gentoo.org/585464

Package-Manager: portage-2.2.28
Closes: https://github.com/gentoo/gentoo/pull/1664
Signed-off-by: Patrice Clement <monsieurp@gentoo.org>