populate-depot: New script to download one or more runtimes

As we start to add more runtimes, we need to decouple the SteamLinuxRuntime depot from any individual runtime. Instead of producing a pre-populated depot as part of a scout build, we can clone this git repository in some convenient place and run ./populate-depot.py to download the necessary files.

Also add convenience Makefile targets to download scout and optionally soldier - these are mostly here as manual tests and as executable examples.

This resolves T17590 and part of T20721.

Signed-off-by: Simon McVittie smcv@collabora.com