Selfupdate to MacPorts 2.8.0 Throws Error 'Couldn't change permissions of the MacPorts sources'

[Steven Smith]

I’m consistently getting this error across multiple boxes when doing a selfupdate to 2.8.0:

> Congratulations, you have successfully upgraded the MacPorts system.
> 
> Error: Couldn't change permissions of the MacPorts sources at /opt/local/var/macports/sources/rsync.macports.org/macports/release/tarballs/base to root: child killed: kill signal

Another selfupdate command ignores the issue, but the ownership of base appears to be broken:

> ls -ld /opt/local/var/macports/sources/rsync.macports.org/macports/release/tarballs/base
> drwxr-xr-x  35 messagebus  polkitd  1120 Oct 22 07:57 /opt/local/var/macports/sources/rsync.macports.org/macports/release/tarballs/base/