fix: dont ignore rpm-ostree when bootc is found

[befanyt]

What does this PR do

This PR should fix #997 where it wouldn't take the rpm-ostree flow when bootc was found on the system, even if it was disabled via the config.

Tested with rpm-ostree=false & bootc=false it used dnf
Tested with rpm-ostree=true & bootc=false it used rpm-ostree
Tested with rpm-ostree=false & bootc=true it used bootc

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself
• If this PR introduces new user-facing messages they are translated

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

This was introducued in PR #986
@tulilirockz maybe you can verify this also works for you.

[SteveLauC]

Thanks for catching and fixing it! Also for the thorough tests!