fix(ios): pod install --project-directory=ios fails

Running `pod install` from outside the the `ios` folder fails because
the path to `React-Codegen` is wrong:

```
% pod install --project-directory=ios
[Codegen] Generating ios/build/generated/ios/React-Codegen.podspec.json
Auto-linking React Native module for target `ReactTestApp`: ReactTestApp-DevSupport
Analyzing dependencies
Fetching podspec for `DoubleConversion` from `../node_modules/react-native/third-party-podspecs/DoubleConversion.podspec`
Fetching podspec for `RCT-Folly` from `../node_modules/react-native/third-party-podspecs/RCT-Folly.podspec`
[!] No podspec found for `React-Codegen` in `ios/build/generated/ios`
```

scripts/react_native_pods.rb

@@ -274,7 +274,7 @@ def generate_temp_pod_spec_for_codegen!(fabric_enabled)
 
   return {
     "spec" => spec,
-    "path" => output_dir,
+    "path" => $CODEGEN_OUTPUT_DIR,  # Path needs to be relative to `Podfile`
   }
 end