For many spkgs, the build/pkgs/SPKG/spkg-build and spkg-install script templates set environment variables before invoking make.

This ticket allows to move environment variable settings common between these scripts to a build/pkgs/SPKG/spkg-env file, which will be included by build/bin/sage-spkg at the top of the generated executable scripts (also named spkg-build and spkg-install).

Moreover, sage-spkg generates an executable script spkg-buildsh (next to spkg-install) that drops the user into a subshell like sage -sh with spkg-env loaded.

This makes it easier to test new versions of packages.

CC: @embray @dimpase @tscrim

Component: build

Work Issues: redo on top of #29096, #29052

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/build_bin_sage_spkg__copy_contents_of_build_pkgs_spkg_spkg_env_to_the_top_of_spkg_build_and_spkg_install @ 60f3b57

Issue created by migration from https://trac.sagemath.org/ticket/29045