Explicitly cargo update in path dependency workspaces.

[obi1kenobi]

Resolves a variant of #167 described here:
#167 (comment)

This issue was observed during a CI run of #900, where an older
tokio-stream version was using a brand-new tokio with stronger
guarantees: several types had newly become UnwindSafe.

Since the baseline tokio-stream was a registry version, it was using
a fresh lockfile with the latest dependencies. The current arm,
meanwhile, was a path dependency fixed to a specific commit, where
cargo had decided to reuse an older version of tokio without the
UnwindSafe trait on those types.

This presented as a (phantom) breaking change: the older tokio-stream's
types were UnwindSafe due to auto-trait propagation from a newer tokio
while the newer tokio-stream with an older tokio did not have that trait.

The solution is to explicitly run cargo update inside path dependency
workspaces we create. This way, both path dependency and index-based
rustdoc JSON generation happens with the latest versions of the target
library's own dependencies.