This uses a build probe to figure out the current toolchain version -
the only alternative to this is bespoke logic in Cargo to tell `test`
when the toolchain is stable/unstable.
The behaviour should be the same as when using
`CFG_DISABLE_UNSTABLE_FEATURES` unless an arbitrary channel is provided
to bootstrap, which I believe necessitates a fork anyway.
Adam Gemmell
dae8ea92a7