krb5: Use sh to run mk_cmds

Otherwise etcupdate apparently can fail if its private object directory
under /var/db is in a filesystem mounted noexec. We shouldn't be
building this target at all, but for now, just apply this workaround.

PR: 291043
Reviewed by: ivy, cy, des
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53861

(cherry picked from commit 673924122470463dfb9089e12b1b4496783a77b8)