[REQUIRED] Environment info

firebase-tools:
15.1.0

Platform:
Windows

[REQUIRED] Test case

Deploy multiple functions co-located in the same source to reuse the same build

[REQUIRED] Steps to reproduce

Deploy multiple functions co-located in the same source to reuse the same build, and observe the build process in cloud build

[REQUIRED] Expected behavior

Different functions sharing the same source reuse the same build, as implemented in #6376
This works in 14.27.0 and earlier

[REQUIRED] Actual behavior

One build per functions even if it shares the same source (and function config as Memory etc)
I would guess this is a regression caused by #9540