populate-depot.py: Update from SteamLinuxRuntime v0.20210806.0

• populate-depot.py: Update from SteamLinuxRuntime v0.20210806.0

  Originally part or all of these commits:

  • steamlinuxruntime@8b0b5a03 "populate-depot: Remove unused support for loading JSON from a file"
  • steamlinuxruntime@084f4e11 "populate-depot: Add more explicit versions of --pressure-vessel"
  • steamlinuxruntime@392878db "Add support for downloading a standalone pressure-vessel release"
  • steamlinuxruntime@4e2a5628 "scout-on-soldier: Fail if toolmanifest.vdf has not been reloaded"
  • steamlinuxruntime@ae46edab "populate-depot: Use a standalone pressure-vessel release by default"
  • steamlinuxruntime@b3f00ee2 "populate-depot: Use latest scout build for LD_LIBRARY_PATH runtime"
  • steamlinuxruntime@5eb43c40 "populate-depot: Store LD_LIBRARY_PATH version in layered scout runtime"
  • steamlinuxruntime@a10a407e "populate-depot: Have an overall version number for the depot as a whole"

  all written by me.

• prepare-test-depots: Don't make populate-depot.py guess

  During CI testing, we know we are working with a pressure-vessel relocatable tarball, so let's specify that explicitly.