share/sendmail: Use consistent ordering when processing files

Otherwise files are handled in the order returned by find(1). This
in turn results in inconsistent ordering of files in the METALOG mtree
file produced by "make installworld -DNO_ROOT", which results in
non-reproducible filesystem images.

Reviewed by: gshapiro, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52349