CI: Add a job to build kani proofs

Currently we do not build the code in the kani tests when PRs are
pushed, instead we run the verifier once a day. We should at least check
the code builds on each PR. One way to do this is to build the proofs
without running them, `kani --only-codegen` does that.

.github/workflows/rust.yml

@@ -163,3 +163,14 @@ jobs:
         env:
           DO_WASM: true
         run: cd hashes && ./contrib/test.sh
+
+  Kani:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: 'Checkout your code.'
+        uses: actions/checkout@v4
+
+      - name: 'Kani build proofs'
+        uses: model-checking/kani-github-action@v1.1
+        with:
+          args: '--only-codegen'