Log Message:
lang/rust: re-enable MAKE_ENV and PKGSRC_MAKE_ENV, and improve comment.

These settings are needed to run the bootstrap kits we produce
using pkgsrc, since pkgsrc prevents the use of $ORIGIN in RPATHs.
This makes us able to run the bootstrap rust compiler from within
the build tree, `pwd`/work/rust-bootstrap/bin/rustc, using the
libraries in `pwd`/work/rust-bootstrap/lib/.