…cal-rebuild

…ce a local-rebuild

If local-rust is the same as the current version, then force a local-rebuild
In Debian, we would like the option to build/rebuild the current release from
*either* the current or previous stable release. So we use enable-local-rust
instead of enable-local-rebuild, and read the bootstrap key dynamically from
whatever is installed locally.
In general, it does not make much sense to allow enable-local-rust without also
setting the bootstrap key, since the build would fail otherwise.
(The way I detect "the bootstrap key of [the local] rustc installation" is a bit hacky, suggestions welcome.)