Install newest build of cargo-outdated

The old version available by default in the GitHub Actions image fails
with:

    thread 'main' panicked at 'package cache lock is not currently held, Cargo forgot to call `acquire_package_cache_lock` before we got to this stack frame', /usr/share/rust/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/cargo-0.57.0/src/cargo/util/config/mod.rs:1542:9
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

.github/workflows/ci.yml

@@ -48,4 +48,5 @@ jobs:
     if: github.event_name != 'pull_request'
     steps:
       - uses: actions/checkout@v2
+      - uses: dtolnay/install@cargo-outdated
       - run: cargo outdated --exit-code 1