fix runghc's GHC detection logic to cover the "in-tree Hadrian build" scenario

Before this patch, runghc would only run the GHC detection logic on Windows and assume that it was invoked through a wrapper script on all other platforms. This patch lifts this limitation and makes that logic work for the scenario where someone is calling the runghc executable directly, without passing an explicit path to GHC.

This fixes #16823 (closed).