Fix calculation related to temporary WAL segment name in basic_archive

Fix calculation related to temporary WAL segment name in basic_archive

The file name used for its temporary destination, before renaming it to
the real deal, has been using a microseconds in a timestamp aimed to be
originally in milli-seconds.  This is harmless as this is aimed at being
a safeguard against name collisions (note MyProcPid in the name), but
let's be correct with the maths.

While on it, add a note in the module's makefile to document why
installcheck is not supported.

Author: Nathan Bossart
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/20221014044106.GA1673343@nathanxps13
Backpatch-through: 15