Add -Wno-error=unused-but-set-variable when building with Clang 13+

This warning triggers many times while building world. Downgrade it to a
warning until all occurrences have been fixed. Once the Clang warnings
have been fixed we should be able to turn it on for GCC as well. See
also f4fed768bba45a406f73ed1491d7e52fd1a8711d which did the same for the
kernel builds.

Reviewed by: arichardson, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D31927