fix detect_server_os in windows runs
fix detect_server_os in windows runs

Change-Id: I6919048fe59b553fa96b72d229afa75a4e1ae731
Reviewed-on: https://review.couchbase.org/c/perfrunner/+/174813
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Bo-Chun Wang <bb86fd206c9c35341feb9b50e74a74a83e254d63@couchbase.com>

couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner