update buildkit

[gilescope]

earth +update-buildkit --BUILDKIT_GIT_SHA=c40c767e201de9ad72c8c52e207dc8f4b5b51e9d

I think lowercase EarthBuild is best here as it works well with go.mod

[github-actions]

🎉 Are we earthbuild yet?

Great progress! You've reduced "earthly" occurrences by 6 (0.11%)

📈 Overall Progress

Branch	Total Count
main	5620
This PR	5614
Difference	-6 (0.11%)

📁 Changes by file type:

File Type	Change
Go files (.go)	➖ No change
Documentation (.md)	➖ No change
Earthfiles	✅ -6

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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.

[gemini-code-assist]

Code Review

This pull request updates repository references and branch names, migrating from the 'earthly' organization and 'earthly-main' branch to 'EarthBuild' and 'main' across documentation, Earthfiles, and build scripts. The review feedback identifies a typo in a variable name in the buildkitd Earthfile and suggests using lowercase 'earthbuild' for consistency in URLs and organization references throughout the codebase.