It seems to be a weird rare issue (none of the links helped me). For me it happens during the installation of www-client/chromium binary package, made with quickpkg. The environment.bz2 is a plain text in my case.

If anybody else running into exactly the same issue, here’s a dirty workaround:

#/etc/portage/make.conf

PORTAGE_BZIP2_COMMAND="/usr/local/sbin/bzip2_or_cat"

#!/bin/bash
#/usr/local/sbin/bzip2_or_cat

FILENAME="${@: -1}"
bzip2 $@ || cat "${FILENAME}"

Also here’s a performance improvement I use (requires app-arch/lbzip2):

#!/bin/bash
#/usr/local/sbin/bzip2_or_cat

FILENAME="${@: -1}"
lbunzip2 -n8 $@ || bzip2 $@ || cat "${FILENAME}"

And don’t forget to chmod +x bzip2_or_cat.