Deny warnings in CI

There are currently a lot of warnings printed in CI, mostly dead code.
Update CI to deny warnings.

Author: tgross35

.github/workflows/main.yml

@@ -1,6 +1,10 @@
 name: CI
 on: [push, pull_request]
 
+env:
+  RUSTDOCFLAGS: -Dwarnings
+  RUSTFLAGS: -Dwarnings
+
 jobs:
   test:
     name: Test

ci/run-docker.sh

@@ -47,7 +47,7 @@ run() {
            "ci/docker/$target"
     docker run \
            --rm \
-           -e RUST_COMPILER_RT_ROOT \
+           -e "RUSTFLAGS=${RUSTFLAGS:-}" \
            -e "CARGO_TARGET_DIR=/builtins-target" \
            -v "$(pwd):/checkout:ro" \
            -w /checkout \