.github/workflows: add some iOS CI coverage

Updates #3812

Change-Id: Ia779c6a2e9a0fd02418bf5479fdb76d4c80c55a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/cross-darwin.yml

@@ -37,6 +37,12 @@ jobs:
         GOARCH: amd64
       run: for d in $(go list -f '{{if .TestGoFiles}}{{.Dir}}{{end}}' ./... ); do (echo $d; cd $d && go test -c ); done
 
+    - name: iOS build most
+      env:
+        GOOS: ios
+        GOARCH: arm64
+      run: go install ./ipn/... ./wgengine/ ./types/... ./control/controlclient
+
     - uses: k0kubun/action-slack@v2.0.0
       with:
         payload: |