Fix: remove doas option (--preserve-env=DIFFPROG)

Standards checklist:

[X] The PR title is descriptive.
[X] The code compiles (cargo build)
[X] The code passes rustfmt (cargo fmt)
[X] The code passes clippy (cargo clippy)
[X] The code passes tests (cargo test)
[X] Optional: I have tested the code myself
- [ ] I also tested that Topgrade skips the step where needed

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