fix(earthfile2llb): make command rename warning thread-safe

[janishorsts]

Fixes #787

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	4050
This PR	4050
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.

[gilescope]

── nits ───────────────────────────────────────────────────

⚠︎ 1 · The concurrency test can't fail. earthfile2llb/interpreter_test.go:44 asserts nothing, and Earthfile:211 runs go test -timeout 5m -json — no
-race. Delete the mutex and this test still passes (the runtime's map-write detector is probabilistic, not a guarantee).

[janishorsts]

── nits ───────────────────────────────────────────────────

⚠︎ 1 · The concurrency test can't fail. earthfile2llb/interpreter_test.go:44 asserts nothing, and Earthfile:211 runs go test -timeout 5m -json — no -race. Delete the mutex and this test still passes (the runtime's map-write detector is probabilistic, not a guarantee).

@gilescope, I'm going to refactor it as this fixes the symptom, not the underlying issue: the grammar deprecation warnings belong to the parsing process.