Trim down the Mac OS CI to just a `cargo build` on libtock_runtime.

The Mac runner took over 1 hour 15 minutes to build QEMU (and it didn't even succeed), so I don't think running `make setup` on Mac OS is reasonable. Shrink its test to `cargo build -p libtock_runtime`, which should be relatively simple and test the most critical items.

In the future, we'll probably want to point it at a more substantial crate, but libtock_runtime is our best option for now.

.github/workflows/mac_os.yml

diff --git a/.github/workflows/mac_os.yml b/.github/workflows/mac_os.yml
index b04b51b..3124b35 100644
--- a/.github/workflows/mac_os.yml
+++ b/.github/workflows/mac_os.yml
@@ -35,5 +35,5 @@
           brew update
           brew install riscv-gnu-toolchain --with-multilib
           cd "${GITHUB_WORKSPACE}"
-          make -j2 setup
-          make -j2 test
+          LIBTOCK_PLATFORM=hifive1 cargo build -p libtock_runtime \
+            --target=riscv32imac-unknown-none-elf