fix(cli): Tag earthbuild/buildkitd binary pointed to by CLI correctly

[kmannislands]

Fixes #335 and #437

Prior to this fix, the released binaries point to the wrong (stagingci) docker repo.

For example from a recent build on main:

$ strings /Users/kieran/Downloads/22e40d18/earth-darwin-arm64 | rg docker.io/earthbuildci/
docker.io/earthbuildci/buildkitd-staging:22e40d18
-ldflags="-X main.DefaultBuildkitdImage=docker.io/earthbuildci/buildkitd-staging:22e40d18 -X main.Version=v0.8.18-22e40d18 -X main.GitSha=22e40d18791c3df67f383e44689d9fa32d5ee5c5 -X main.DefaultInstallationName=earth "
-ldflags="-X main.DefaultBuildkitdImage=docker.io/earthbuildci/buildkitd-staging:22e40d18 -X main.Version=v0.8.18-22e40d18 -X main.GitSha=22e40d18791c3df67f383e44689d9fa32d5ee5c5 -X main.DefaultInstallationName=earth "

These get copied as-is by the release automation on release (not rebuilt) per the plan in #76

Related almost-fixes:
#192
#219

I'll do a full release test of this branch and we should see the CLI binary produced by CI pointing to docker.io/earthbuild/buildkitd:v0.8.18-<hash> and the warning referenced in #335 and #437 go away

[gemini-code-assist]

Code Review

This pull request updates the DEFAULT_BUILDKITD_IMAGE in release/Earthfile to use the $VERSION variable instead of $RELEASE_TAG. While this change correctly identifies the intended version, a critical issue was identified regarding argument propagation: the VERSION and DEFAULT_BUILDKITD_IMAGE arguments are not declared or passed down through the +all-binaries target and its dependencies in the root Earthfile, meaning the changes will be ignored during the build process.

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	5557
This PR	5557
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.