Fix RNTester Podfile.lock update in release workflow (#43513)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43513

The previous `update_podfile_lock.sh` script would fail as executed from the repo root (could not locate RNTester dir). Delete this and replace with direct calls in `prepare-package-for-release.js`, which will fail script on error.

 {F1469216632}

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D54949214

fbshipit-source-id: 4f032069e803e84f835c279d01332d16787dfafc