default-versions: add rust-bootstrap to RUST section

[ngie]

Summary

lang/rust-bootstrap is another potential option for RUST.
document the value in the comment so others might consider its use.

lang/rust-bootstrap has more tunable knobs which allow for more minimal
builds of the rust toolchain than lang/rust -- which may be desirable
for appliance or iterative builds.

[allanjude]

Reviewed-by: allanjude