mac_pw_pool: fix DHSTATE default falling back to $PWSTATE

[ROKUMATE]

Looks like a leftover from the rename in 8aa0ffd — that commit wrote line 29 using the old
variable name, and e427ad3 added the real PWSTATE at line 39 shortly after.

Nothing sets PWSTATE in the environment today .. so the fallback always fires and DHSTATE
already resolves to dh_status.txt ... no change in current behavior ... but line 39 does support
overriding PWSTATE, and if you use it both variables end up pointing at the same file ...
LaunchInstances.sh:307-310 and SetupInstances.sh:507-510 each do
cp "$X" "${X}~" then mv tmp "$X", so host state would land in pw_status.txt and
SetupInstances.sh:108-128 would read host records as instance records.

Side effect: this also clears the SC2034 DHSTATE appears unused shellcheck warning on that line
(20 findings in the file, now 19).