Switch the Mac OS CI workflow to libtock2.

Building `libtock2` is a more thorough test that building `libtock_runtime`, as `libtock2` depends on `libtock_runtimes` (and eventually many other crates).

Author: Johnathan Van Why

.github/workflows/mac-os.yml

@@ -24,5 +24,5 @@ jobs:
       - name: Build and Test
         run: |
           cd "${GITHUB_WORKSPACE}"
-          LIBTOCK_PLATFORM=nrf52 cargo build -p libtock_runtime \
+          LIBTOCK_PLATFORM=nrf52 cargo build -p libtock2 \
             --target=thumbv7em-none-eabi