Makefile.inc1: Always use ${PKG_CMD}

Previously we had a mix of ${PKG_CMD} and bare 'pkg', which is
wrong, and breaks the build when 'pkg' isn't in the tools path,
e.g. when cross-building.

MFC after: 2 weeks
Reviewed by: wosch, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56655