devel/avro-cpp: fix const bugs in custom formatters

These will break with fmt 11.2.

Since the project is broken in other ways, do only the minimum
so it will survive a feature-incomplete inefficient "TRYBROKEN=yes"
build, and then mark it BROKEN with its bugs.

PR: 286858
Reported by: diizzy@
Approved by: maintainer timeout (sunpoet@, 82 days)