Fail CI on bad intradoc links

I'm looking at extending the docs and want to make sure I don't
introduce a bad link.

It looks like
- CI won't fail on bad intradoc links
- We can't get it to with the `cargo` action

So dropping down to the shell to make sure bad links error, instead of
warn.

.github/workflows/ci.yml

@@ -135,10 +135,9 @@ jobs:
           override: true
 
       - name: Build
-        uses: actions-rs/cargo@v1
-        with:
-          command: doc
-          args: --verbose --features "std docsrs"
+        env:
+          RUSTDOCFLAGS: -D warnings
+        run: cargo doc --no-deps --document-private-items --workspace --verbose --features "std docsrs"
 
   fmt:
     name: Check formatting