Pass --locked when installing rustfmt in integration tests

There was recently an issue where `cargo install` was installing a newer
version of a dependency than the one listed in our Cargo.toml. The newer
version added deprecation warnings that caused our continuous integration
tests to break.

As mentioned in the `cargo help install` docs, passing the `--locked`
flag should force cargo to use the `Cargo.lock` file included with
the repository.

ci/integration.sh

@@ -15,7 +15,7 @@ set -ex
 # it again.
 #
 #which cargo-fmt || cargo install --force
-CFG_RELEASE=nightly CFG_RELEASE_CHANNEL=nightly cargo install --path . --force
+CFG_RELEASE=nightly CFG_RELEASE_CHANNEL=nightly cargo install --path . --force --locked
 
 echo "Integration tests for: ${INTEGRATION}"
 cargo fmt -- --version