west build: fix a leftover "log" reference in build.py

A recent commit changed all references to the global 'west.log' instance
(now deprecated) to the new WestCommand logging API, but another PR
merged in the same period added an extra instance that is now causing CI
to fail. Convert this last reference to the new API.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>