This fixes the build issue identified in #5664 at the toolchain level
rather than working around it in our own build. The next step in fixing
this will be to remove the nightly override in the future when the
toolchain becomes stable.
This allows the `wasmtime` binary provided in our release artifacts to
cross-compile: `wasmtime compile` can build a `.cwasm` for any platform
that Wasmtime supports, not just the host platform. This may be useful
in some deployment scenarios.
We don't turn on `all-arch` by default because it increases build time
and binary size of Wasmtime itself, and other embedders of the
`wasmtime` crate won't necessarily want this; hence, we set it only as
part of the CI build configuration.
Fixes#5655.
