rustbuild: Don't build target compilers in stage0

rustbuild: Don't build target compilers in stage0

The `doc-book` and `doc-nomicon` steps accidentally depended on a rustbook
compiled by a cross-compiled compiler, which isn't necessary. Be sure to set the
`host` on these dependency edges to the build compiler to ensure that we're
always using a tool compiled for the host platform.

This was discovered trawling the build logs for the new dist bots and
discovering that they're building one too many compilers in stage0.