Mk: fix checksum/fetch/makesum with large distfiles set

When a port has a very large set of DISTFILES, makesum/fetch dies with
an obscure message such as:

/tmp/makeQEBorT: /usr/bin/env: Argument list too long

The interface from make(1) to sh(1) passes the list of MASTER_SITES and
DISTFILES as arguments on the command line. At one point, it reaches
ARG_MAX and fails with the previous message. This changes the interface
to use temporary files in WRKDIR.

PR: 295029
Differential Revision: https://reviews.freebsd.org/D59475