Add postInstall step to restore the executable bit on scripts.

[tom-un]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

The react-native-macos npm publish step is performed on a Windows Azure DevOps agent. Windows file systems don't have an executable bit, so the npm package doesn't contain the executable bit on bash scripts.

Long term we will change the build pipeline to use a Mac or Linux build agent. But in the near term the build pipeline still has some other Windows dependencies so for now we will use an npm postinstall script to restore the executable bit on scripts.

Microsoft Reviewers: Open in CodeFlow