PortageEnvironment does not support repos.conf

[MageSlayer]

Paludis bug http://paludis.exherbo.org/trac/ticket/1347
Corresponding Gentoo bug
https://bugs.gentoo.org/show_bug.cgi?id=658268

[l29ah]

For the record: the symptom is

cave@1582691195: [WARNING portage_environment.dodgy] Use of Portage configuration files will lead to sub-optimal performance and loss of functionality. Full support for Portage configuration formats is not guaranteed; issues should be reported via trac.

Error:
  * In program cave resolve world:
  * When making environment from specification '':
  * When creating PortageEnvironment using config root '':
  * PORTDIR empty or unset (paludis::portage_environment::PortageEnvironmentConfigurationError)