Reduce code duplication by calling fetch target

when converting the do-fetch target to proper scripting
we lost the ability to overwrite do-fetch when running make makesum.
as reported here: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=

Let's call again do-fetch directly instead of duplicating its content

PR: 215530
Reported by: jbeich
Differential Revision: https://reviews.freebsd.org/D21544