Explicitly install clippy during CI

It used to be included in the rust:latest container, but apparently no
longer is.

.cirrus.yml

@@ -33,6 +33,7 @@ test_task:
     - fi
   clippy_script:
     - if rustc --version | grep -q nightly; then
+    -   rustup component add clippy
     -   cargo clippy --all-features --all-targets
     - fi
   audit_script: