fix(ios, app): macOS 12.3 removed python, use python3

[mikehardy]

Description

macOS 12.3 (released March 24, about a month ago) removed the python2 installation (binary of python)
Our ios config script depended on that binary existing and silently failed / false-positived if it did not exist, with symptom being that on ios the config used all default values.

I do a check for python3 existence now and forward-ported the python2 code to python3 so our firebase.json parsing works again. Likely needs the same thing for react-native-google-mobile-ads...

This was only visible on macOS 12.3 machines running our e2e harness (where this functionality is probed) or by eagle-eyed developers who noticed the problem. A related change will move CI to macos-12 runners (just now available!) to make sure it fails on them, then I'll rebase against this to make sure it works...

Related issues

Fixes #6226
Fixes #6203

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

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Name	Status	Preview	Updated
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[codecov]

Codecov Report

Merging #6234 (593de24) into main (f26482d) will not change coverage.
The diff coverage is n/a.

❗ Current head 593de24 differs from pull request most recent head 258b267. Consider uploading reports for the commit 258b267 to get more accurate results

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  Complexity      643      643           
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  Files           208      208           
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  Branches       1633     1633           
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  Hits           5501     5501           
  Misses         4543     4543           
  Partials        258      258