pgsql: Avoid package qualification of $windows_os

Avoid package qualification of $windows_os

Further fallout from commit 6ee26c6a4b. To keep code in sync and avoid
issues on older releases with different package names, simply use the
unqualified name like many other places in our code.

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b5025a2e339363b621c099ca676baa186314cdb5

Modified Files
--------------
src/bin/pg_rewind/t/003_extrafiles.pl | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)