[iOS] - False positive when generating Variants.swift

[arthurpalves]

Environment

Variants:

• Way of installation: Any
• Version 1.2.1

Your machine:

• OS: MacOS
• Processor Apple Silicon

Project's platform:

• Platform: iOS

Describe the bug

This relates to #205, when we rely on custom configs whose env option is set to true and python 2.7 isn't installed.
stdout states that Variants.swift has been generated successfully but it isn't.

Steps to reproduce

1. Do not have Python 2.7 installed.
2. Have at least 1 custom config setting env: true (regardless of the env var existing or not)
3. Execute switch command to that variant.
4. Variants prints to stdout that ⚙️ '[...]/Variants/Variants.swift' has been generated with success.
5. Variants/ folder doesn't contain Variants.swift.

Expected behavior
A clear error message stating what happened and the steps to fix it.