If PrepareTempTablespaces() has never been called in the current
transaction, OpenTemporaryFile() will fall back to using the default
tablespace, which is a bug if the user wanted temp files placed elsewhere.
gistInitBuildBuffers() appears to have this disease already, and it
seems like an easy trap for future coders to fall into.

We discussed other ways to close this gap, but none of them are prettier
or more reliable than just having BufFileCreateTemp do it.  In particular,
having fd.c do this creates layering issues that we could do without.

Per suggestion from Melanie Plageman.  Arguably this is a bug fix, but
nobody seems very excited about back-patching, so change in HEAD only.

Discussion: https://postgr.es/m/CAAKRu_YwzjuGAmmaw4-8XO=OVFGR1QhY_Pq-t3wjb9ribBJb_Q@mail.gmail.com