fix(os): detect Fedora IoT Edition as immutable Fedora variant

[d-k-bo]

Without this change, it is detected as a regular Fedora variant and updating fails because neither dnf nor yum is found.

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
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[SteveLauC]

Thanks!

[d-k-bo]

Thanks for merging!