Fix warning on v0.8.17 release about unsupported version of buildkit

[kmannislands]

When running the v0.8.17 release as-is, you get a warning on bootstrap and subsequent builds:

❯ ~/Downloads/earth-darwin-arm64 bootstrap
           buildkitd | Starting buildkit daemon as a docker container (earth-buildkitd)...
           buildkitd | ...Done
           buildkitd | Using a non-EarthBuild version of Buildkit. This is not supported.
           bootstrap | Bootstrapping successful.

Everything works but the warning is a bit off-putting

NB: This is possibly due to a case-sensitive comparison around this area of code.