populate-depot: Use latest scout build for LD_LIBRARY_PATH runtime

If we're building a SteamLinuxRuntime_soldier depot and we want to test launching it from a LD_LIBRARY_PATH runtime (as Steam would), there is no point in trying to match the version number of scout to a version number of soldier. Worst case it'll just fail, and best case it'll use an arbitrary scout build that happens to have been done on the same day as the soldier build.

This is particularly problematic right now because the latest build of soldier happens to be 0.20210618.0. scout build 0.20210618.0 existed, but had a broken steam-runtime.tar.xz, because the setup.sh script is not executable.

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I'm pretty sure this is right, but I want to get a branch that contains it to pass CI before I say that more definitely.