${EPREFIX}/usr/$(get_libdir)/portage/bin/setsid has interpretor set to: #!/usr/bin/python -O ... which is broken for prefix installs. /usr/bin/env cannot be used in this instance if the "-O" option is required, so this path will have to be altered during the package installation process. All of the other binaries in the 'bin' directory have the hard-coded interpretor (for my system) '/opt/gentoo/usr/bin/python -O'.
thanks, I fixed that already, will release tonight.
this is fixed in 2.2.00.13878, thanks!