Build dummy libcommunicator into the 'neon' extension (#12266)

This doesn't do anything interesting yet, but demonstrates linking Rust
code to the neon Postgres extension, so that we can review and test
drive just the build process changes independently.

.github/workflows/build-macos.yml

@@ -99,7 +99,9 @@ jobs:
       # toes, ensure that the toolchain is up-to-date beforehand.
       - name: Update rust toolchain
         run: |
-          rustup update
+          rustup --version &&
+          rustup update &&
+          rustup show
 
       - name: Cache cargo deps
         uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4.2.3