Add variant mechanism to build temporary packages.HEADmaster

Build some variants for gnome, to demonstrate how:
- nopulse
- an "I'm feeling lucky" mode that randomly attempts to build one of the
  ten thousand packages with stable keywords that isn't yet installed
  via autounmask
- compile some unstable compilers via --buildpkgonly. Nothing is ever
  installed or can mix with the system, so it cannot ever have an effect
  on other binhost packages. But users can install upcoming compilers
  for testing.

Variant builds are built in an ephemeral nspawn container based on the
original one. The primary build has to succeed in order to build a
variant, otherwise we assume resolution fails anyway. The ephemeral
container means we can freely modify the portage config, even in totally
broken ways, and our changes get automatically wiped after exiting. The
only persistent data is the binpkgs cache, which is mounted from the
real location.

Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>

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