Tokio has had to be pinned, because the 1.17 release introduces
a dependency on windows_sys, which fails to build on Windows on
Bazel.
The issue appears to be the build script of a subcrate - it is using
CARGO_MANIFEST_DIR to update the linking path so windows.lib can be
found (it's contained in that crate), but the path is set incorrectly.
dfc25285a2/crates/targets/x86_64_msvc/build.rs
One way we might be able to work around it is to add to the link path
in our own build script.
