Update firestore podspec presubmit to ios platform (#7998)

Firestore takes over an hour on presubmit.
https://github.com/firebase/firebase-ios-sdk/runs/2456478584?check_suite_focus=true

This is to limit the platform and intend to shorten the presubmit processing time.

Author: granluo

.github/workflows/firestore.yml

@@ -259,4 +259,4 @@ jobs:
     - name: Setup Bundler
       run: scripts/setup_bundler.sh
     - name: Build and test
-      run: scripts/third_party/travis/retry.sh pod spec lint FirebaseFirestore.podspec --skip-tests --allow-warnings --sources='https://github.com/firebase/SpecsTesting','https://github.com/firebase/SpecsDev.git','https://github.com/firebase/SpecsStaging.git','https://cdn.cocoapods.org/'
+      run: scripts/third_party/travis/retry.sh pod spec lint FirebaseFirestore.podspec --skip-tests --allow-warnings --platforms=ios --sources='https://github.com/firebase/SpecsTesting','https://github.com/firebase/SpecsDev.git','https://github.com/firebase/SpecsStaging.git','https://cdn.cocoapods.org/'