Suggested cache set is insufficient for cargo install

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From the “Cargo Home” docs page (live, source), referring to what
directories should be cached in CI:

It should be sufficient to only cache the following directories across builds:

• bin/
• registry/index/
• registry/cache/
• git/db/

This doesn’t seem to be sufficient when the job runs cargo install to
install tools, like Cargo subcommands (e.g., cargo-raze). If a job
installs a binary and only caches these directories, a re-run of the job
will fail at cargo install time with

error: binary `cargo-raze` already exists in destination
Add --force to overwrite

because the binary exists but Cargo has no record that it has been
installed. Sample failure:
https://github.com/tensorflow/tensorboard/runs/1367174061

It sufficed for me to also cache .crates.toml and .crates2.json.
Maybe only the former is needed; I included the latter to be safe.
Especially since these are dotfiles, it would be nice to document that
they, too, should be cached.

I would submit a patch, but I don’t know exactly what the right
recommendation is.

---

cargo version 1.47.0 on Linux (Debian/Ubuntu)

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@rustbot modify labels: A-documenting-cargo-itself

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