refactor: enable automatic iOS builds (#209)

* refactor: enable automatic iOS builds

Part of https://github.com/ooni/probe/issues/1335

* fix: go mod tidy

internal/engine/.github/workflows/ios.yml

@@ -1,18 +0,0 @@
-name: ios
-on:
-  push:
-    branches:
-      - mobile-staging
-      - 'release/**'
-jobs:
-  test:
-    runs-on: macos-latest
-    steps:
-      - uses: actions/setup-go@v1
-        with:
-          go-version: "1.14"
-      - uses: actions/checkout@v2
-      - run: ./build-ios.bash
-      - run: ./publish-ios.bash
-        env:
-          BINTRAY_API_KEY: ${{ secrets.BINTRAY_API_KEY }}