bin/fmt: remove use of `cargo fmt`
bin/fmt: remove use of `cargo fmt`

This simplifies our formatting script so that it does not need any of
the Cargo metadata to run; the new version just finds all Rust files
(*.rs) and runs `rustfmt` on them individually.

BUG=None
TEST=bin/fmt
TEST=bin/fmt --check

Change-Id: I45aaee497d4f1dfcc03b3e2c5c2f27feb974dc80
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1865371
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm