Handle MAKEFLAGS in sage-spkg

[orlitzky]

In #33125 we add special MAKEFLAGS parsing to build/make/Makefile.in because the alternative would require us to modify spkg-install for all script packages. But after #29386, script packages will be handled by sage-spkg, so it makes sense to reconsider moving the MAKEFLAGS handling into sage-spkg.

This is what the documentation recommends: https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html

And while the end result should be the same, handling MAKEFLAGS in the script would be a bit more portable and simpler from the caller's perspective.

Depends on #29386

CC: @mkoeppe

Component: build

Issue created by migration from https://trac.sagemath.org/ticket/33136