Implement loading of environment.bz2 for the pkg_info() phase.

Implement loading of environment.bz2 for the pkg_info() phase.
Since a temporary directory is required for processing of
${T}/environment, and we want a user who's not in the portage
group to be able to run the pkg_info() phase, PORTAGE_TMPDIR
is temporarily overridden with a directory created by mkdtemp.
To make this work, doebuild() creates the tempdir and cleans
it up in a finally block. (trunk r8649)

svn path=/main/branches/2.1.2/; revision=8650