ci: add bitstream synth to CI

.github/workflows/main.yml

@@ -17,4 +17,15 @@ jobs:
         mkdir build
         cd build
         cmake ..
-        make
+        make
+        cd ../../..
+
+    # nix for FPGA deps
+    - uses: cachix/install-nix-action@v16
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+
+    - name: ice40 synth
+      run: |
+        cd firmware
+        nix-shell --command 'make top.bin'