Add quotes around paths in shell script call (#29160)

Summary:
Without this the RNTester app on iOS can’t be built in a folder that has a space in the path.

## Changelog

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[iOS] [FIXED] - RNTester app builds in a path that contains a space
Pull Request resolved: https://github.com/facebook/react-native/pull/29160

Test Plan:
1. Clone react-native into a folder where the path has a space
2. Follow the instructions at https://github.com/facebook/react-native/blob/master/RNTester/README.md to get the RNTesterPods.xcworkspace
3. Open the workspace and build the project. Path to workspace was /Users/richard/UX Cam/Code/Test Projects/ReactNative/react-native/RNTester/RNTesterPods.xcworkspace

4. See error message - '/Users/richard/UX' is the first part of my path before the space

/Users/richard/Library/Developer/Xcode/DerivedData/RNTesterPods-ctjcmrlwzntpssgmcjydmovoztwl/Build/Intermediates.noindex/RNTesterPods.build/Debug-iphonesimulator/RNTester.build/Script-68CD48B71D2BCB2C007E06A9.sh: line 3: /Users/richard/UX: No such file or directory

Reviewed By: RSNara

Differential Revision: D22118259

Pulled By: TheSavior

fbshipit-source-id: 1213d88f66fd47dd052eaeb5f26da9056887f5ea

RNTester/RNTesterPods.xcodeproj/project.pbxproj

@@ -586,7 +586,7 @@
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 			shellPath = /bin/sh;
-			shellScript = "export NODE_BINARY=node\nPROJECT_ROOT=$SRCROOT/.. $SRCROOT/../scripts/react-native-xcode.sh RNTester/js/RNTesterApp.ios.js\n";
+			shellScript = "export NODE_BINARY=node\n\"$SRCROOT/../scripts/react-native-xcode.sh\" RNTester/js/RNTesterApp.ios.js\n";
 		};
 		C1C7A9D58CE1D09515F20577 /* [CP] Copy Pods Resources */ = {
 			isa = PBXShellScriptBuildPhase;