scout-on-soldier: Add a convenience script for developers
It's convenient to be able to use a command like
.../SteamLinuxRuntime_soldier/run -- xterm
to enter the soldier environment for some quick testing, but there isn't a direct equivalent for scout-on-soldier. Add one.
We have to make an assumption about the location of SteamLinuxRuntime_soldier, because officially only Steam knows where to find each compatibility tool; but we can guess that most developers will have put SteamLinuxRuntime_soldier and SteamLinuxRuntime in the same Steam library, whether that's the default or not.
Suggested by @timo on chat.