feat(ios, sdk): allow FIREBASE_SDK_VERSION override

This should make it easier for Expo folks to override the SDK when
they need to

docs/index.md

@@ -310,6 +310,8 @@ $FirebaseSDKVersion = '11.5.0'
 
 Once changed, reinstall your projects pods via pod install and rebuild your project with `npx react-native run-ios`.
 
+Alternatively, if you cannot edit the Podfile easily (as when using Expo), you may add the environment variable `FIREBASE_SDK_VERSION=11.5.0` (or whatever version you need) to the command line that installs pods. For example `FIREBASE_SDK_VERSION=11.5.0 yarn expo prebuild --clean`
+
 ### Increasing Android build memory
 
 As you add more Firebase modules, there is an incredible demand placed on the Android build system, and the default memory

packages/app/RNFBApp.podspec

@@ -27,6 +27,11 @@ Pod::Spec.new do |s|
   # React Native dependencies
   s.dependency          'React-Core'
 
+  if (ENV.include?('FIREBASE_SDK_VERSION'))
+    Pod::UI.puts "#{s.name}: Found Firebase SDK version in environment '#{ENV['FIREBASE_SDK_VERSION']}'"
+    $FirebaseSDKVersion = ENV['FIREBASE_SDK_VERSION']
+  end
+
   if defined?($FirebaseSDKVersion)
     Pod::UI.puts "#{s.name}: Using user specified Firebase SDK version '#{$FirebaseSDKVersion}'"
     firebase_sdk_version = $FirebaseSDKVersion