[ci] Pin Kani to last known-good version (#930)

* [ci] Pin Kani to last known-good version

Makes progress on #929

* Update ci.yml

.github/workflows/ci.yml

@@ -321,6 +321,8 @@ jobs:
           # Go back to testing all features once the Kani GitHub Action supports
           # specifying a particular toolchain.
           args: "--package zerocopy --features __internal_use_only_features_that_work_on_stable --output-format=terse --randomize-layout --memory-safety-checks --overflow-checks --undefined-function-checks --unwinding-checks"
+          # TODO(#929): Either roll this automatically or don't pin.
+          kani-version: 0.46.0
 
   check_fmt:
     runs-on: ubuntu-latest