Modified ExecutableForPlatform to give correct extension to executable.
Modified ExecutableForPlatform to give correct extension to executable.

For agent copies, we didn't care about the file extension of the
executable produced by ExecutableForPlatform, but for executing agents
in tunnel hosts, it's easier if the extracted agent has the correct
extension.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen