This changes the `test` build script so that it does not use the default
fingerprinting mechanism in cargo which causes a full scan of the
package every time it runs. This build script does not depend on any of
the files in the package.
This is the recommended approach for writing build scripts.
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.