Fix message when there are no buildable packages (#11722)
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## Summary
I noticed that when running "uv build --all-packages" in an empty
workspace with no buildable packages, it reports that there are
buildable packages. Which I believe to be an error in the message. This
patch fixes the typo. I did not find any relevant issues.
## Test Plan
I've verified, to the best of my ability, that this did not introduce
any additional errors in related existing tests. Considering the nature
of the change I believe it's sufficient.
