ci: print bindgen version used for rustls.h diff

The existing CI has a step where it regenerates `rustls.h` using `make
src/rustls.h`, which in turn invokes `cbindgen`.

I'm seeing a diff created in CI that I don't see locally and suspect the
reason is a difference in `cbindgen` versions. To help debug this now
and into the future, this commit updates CI to print its `cbindgen`
version before performing the diff check.

Author: cpu

.github/workflows/test.yaml

@@ -102,6 +102,7 @@ jobs:
         with:
           persist-credentials: false
       - run: touch src/lib.rs
+      - run: cbindgen --version
       - run: make src/rustls.h
       - run: git diff --exit-code