Generate ip.txt before SKIP_BUNDLING check

[keatongreve]

Fixes#20553.

Re-applies the change as described in #16533, which was closed for being stale. Thanks to @emusgrave for the original PR/change.

Test Plan:

1. Ensure build directory clean
2. Run a debug build targeting a physical iOS device
3. Verify application loads JS bundle via a packager URL, and does not attempt to load a local pre-packaged JS bundle

Release Notes:

Help reviewers and the release process by writing your own release notes. See below for an example.

[INTERNAL] [BUGFIX] [./scripts] - Enable physical devices to still load from packager when SKIP_BUNDLING is set for clean Debug builds

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[react-native-bot]

This pull request was closed by @keatongreve in 9c1ea45.

Once this commit is added to a release, you will see the corresponding version tag below the description at 9c1ea45. If the commit has a single master tag, it is not yet part of a release.