Adjust Nimbus FML CLI to only import R when it is required

[jeddai]

Pull Request checklist

• Quality: This PR builds and tests run cleanly
  • Note:
    • For changes that need extra cross-platform testing, consider adding [ci full] to the PR title.
    • If this pull request includes a breaking change, consider cutting a new release after merging.
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGES_UNRELEASED.md or an explanation of why it does not need one
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Branch builds: add [ff-android: firefox-android-branch-name] and/or [fenix: fenix-branch-name] to the PR title.

[jhugman]

I've r+'d here. There are some comments, but I've added suggestions for all of them.

Thanks!

[codecov-commenter]

Codecov Report

Base: 46.44% // Head: 46.44% // No change to project coverage 👍

Coverage data is based on head (c2ff14b) compared to base (a812bfa).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5361   +/-   ##
=======================================
  Coverage   46.44%   46.44%           
=======================================
  Files         169      169           
  Lines       14343    14343           
=======================================
  Hits         6661     6661           
  Misses       7682     7682           

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