What is happening on buildfarm member baiji?

The last two runs on baiji have failed at the installcheck stage,
with symptoms that look a heck of a lot like the most recent system
catalog changes haven't taken effect (eg, it doesn't seem to know
about pg_type.typarray).  Given that the previous "check" step
passed, the most likely explanation seems to be that some part
of the "install" step failed --- I've not tried to reproduce the
behavior but it looks like it might be explained if the install
target's postgres.bki file was not getting overwritten.  So we
have two issues: what exactly is going wrong (some new form of
Vista brain death no doubt), and why isn't the buildfarm script
noticing?
        regards, tom lane