Fix unstable RCTAppDelegate podspec

Summary:
This change should fix [#39971](#39971), computing the relative path from the App path to the pod installation root and using that instead of the absolute path to the `react-native.config.js` file

## Changelog
[Internal] - Stabilize RCTAppDelegate podspec

Differential Revision: D50323710

packages/react-native/Libraries/AppDelegate/React-RCTAppDelegate.podspec

@@ -117,6 +117,8 @@ Pod::Spec.new do |s|
     s.dependency "React-debug"
     s.dependency "React-rendererdebug"
 
+    relative_path_from = Pathname.new(ENV['APP_PATH']).relative_path_from(Pod::Config.instance.installation_root)
+
     s.script_phases = {
       :name => "Generate Legacy Components Interop",
       :script => "
@@ -125,7 +127,7 @@ source $WITH_ENVIRONMENT
 ${NODE_BINARY} ${REACT_NATIVE_PATH}/scripts/codegen/generate-legacy-interop-components.js -p #{ENV['APP_PATH']} -o ${REACT_NATIVE_PATH}/Libraries/AppDelegate
       ",
       :execution_position => :before_compile,
-      :input_files => ["#{ENV['APP_PATH']}/react-native.config.js"],
+      :input_files => ["#{relative_path_from}/react-native.config.js"],
       :output_files => ["${REACT_NATIVE_PATH}/Libraries/AppDelegate/RCTLegacyInteropComponents.mm"],
     }
   end