Use go install for dep binaries when building docker

[beautifulentropy]

Update the way our docker build script installs the binaries we rely on
at runtime to avoid "go get" deprecation warnings.

Fixes #5744

[beautifulentropy]

Oof, I can repro these failures locally. Pushing back to draft.

[aarongable]

My working hypothesis is that pebble-challtestsrv@latest is somehow not the same as what we were getting before, maybe thanks to pebble "not properly supporting modules". But I didn't have the ability to dig into that very well, obvs :)

[aarongable]

Thanks for picking this up!