CI: Test Rust both with hello-world and gcoap example

Build failures after #189 show
that the test coverage is insufficient to catch C2Rust breakage; this
mitigates it by building an example that pulls in much more code.

.github/workflows/build.yml

@@ -141,6 +141,7 @@ jobs:
           # action does only minimal fetching.
           (cd RIOT && git fetch origin master && git checkout FETCH_HEAD)
           make -CRIOT/examples/rust-hello-world BUILDTEST_MAKE_REDIRECT='' buildtest
+          make -CRIOT/examples/rust-gcoap BUILDTEST_MAKE_REDIRECT='' buildtest
           (cd RIOT && git switch -)
         env:
           BUILD_IN_DOCKER: 1