ports-mgmt/sccache-overlay: Add ports overlay for sccache

Sccache is a compiler cache but unlike ccache it comes with Rust
support. This package provides a standalone version of sccache
that can work in Poudriere jails with glue for hooking it up to
the ports framework by simply activating an overlay.

Based on tcberner's https://reviews.freebsd.org/D31243