Provides the target operating system for the current build stage.
DRONE_STAGE_OS=linux
List of all possible values:
DRONE_STAGE_OS=darwin
DRONE_STAGE_OS=dragonfly
DRONE_STAGE_OS=freebsd
DRONE_STAGE_OS=linux
DRONE_STAGE_OS=netbsd
DRONE_STAGE_OS=openbsd
DRONE_STAGE_OS=solaris
DRONE_STAGE_OS=windows