Add objc build to gha

Author: morganchen12

.github/workflows/sample.yml

@@ -41,3 +41,29 @@ jobs:
             -destination 'platform=iOS Simulator,OS=latest,name=iPhone 11 Pro' \
             clean build test \
             ONLY_ACTIVE_ARCH=YES
+
+    build:
+    name: ObjC build
+    runs-on: macOS-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@master
+      - name: Setup
+        run: |
+          cd samples/objc
+          gem install bundler
+          bundle install
+          gem install xcpretty
+          bundle exec pod install --repo-update
+      - name: Build
+        # Note: tests aren't run here because the ObjC sample has no test targets.
+        run: |
+          cd samples/objc
+          xcodebuild \
+            -workspace FirebaseUI-demo-objc.xcworkspace \
+            -scheme FirebaseUI-demo-objc \
+            -sdk iphonesimulator \
+            -destination 'platform=iOS Simulator,OS=latest,name=iPhone 11 Pro' \
+            clean build \
+            ONLY_ACTIVE_ARCH=YES