According to this PR, support for the new text-based lockfile in bun should have been added.
I have deployed a fresh create-next-app project using the text-based lockfile (bun.lock) and the file was not automatically detected, hence npm was used for the build.
The issue can be solved via a custom installCommand, however it should not be needed, as automatic detection should be present as per the aforementioned PR
btw Happy new Year everyone!