pgsql: Fix calculation related to temporary WAL segment name in basic_a

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

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9ebcb5ffdf3a1f49388c38ba5273370f49bf7d19

Modified Files
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contrib/basic_archive/Makefile | 3 ++-
contrib/basic_archive/basic_archive.c | 4 ++--
2 files changed, 4 insertions(+), 3 deletions(-)