stage 0 doctests fail: the `-Z unstable-options` flag must also be passed to enable the flag `crate-version`

[RalfJung]

Running libstd doc tests in stage 0 worked for some time, but now it is broken again:

$ ./x.py test src/libcore --stage 0
[...]
   Doc-tests core
error: the `-Z unstable-options` flag must also be passed to enable the flag `crate-version`

error: test failed, to rerun pass '--doc'


command did not execute successfully: "/home/r/src/rust/rustc.3/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "8" "--release" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/home/r/src/rust/rustc.3/src/libtest/Cargo.toml" "-p" "core" "--" "--quiet"
expected success, got: exit code: 1


failed to run: /home/r/src/rust/rustc.3/build/bootstrap/debug/bootstrap test src/libcore --stage 0

[RalfJung]

This work snow, probably things migrated to to the bootstrap rustdoc. I suspect this can come back any time in the future when a new flag gets introduced (Cc @rust-lang/rustdoc), but for now it can be closed.