Peer authentication failure with postgres tests on ppc64 only?

I have a number of packages which use %postgresql_tests_run to start
a postgres server in %check and they all seem to have failed during
the mass rebuild, for example:

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The failure is in %postgresql_tests_run when, after it has started
the server it tries to create a database:

+ createdb -h /tmp mockbuild --owner mockbuild -p 36884
BUILDSTDERR: createdb: could not connect to database template1: FATAL:
Peer authentication failed for user "mockbuild"

That basically means that createdb connected to the server using a
unix domain socket but the server didn't see the credentials on the
socket identifying the remote party as mockbuild correctly.

This is only happening on ppc64le so I'm guessing at some sort of
kernel problem on the ppc64le builders unless somebody has another
suggestion as to what is going on?