bin/ebuild: fix --color=n for bug #531690

When the NOCOLOR variable is overridden in portage.settings, it's
necessary to call the config.backup_changes method in order for the
change to persist through config.reset calls.

X-Gentoo-Bug: 531690
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=531690
Acked-by: Brian Dolbec <dolsen@gentoo.org>