Fix overcomplicated Guix step

[GideonBear]

What does this PR do

Closes #1289

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@promisedlandt Could you test if this solves the issue?
cargo install --git https://github.com/GideonBear/topgrade --branch fix-guix
~/.cargo/bin/topgrade --only guix --dry-run and ~/.cargo/bin/topgrade --only guix

[promisedlandt]

Both dry-run and wet-run work as expected with this change.

Completely unrelated: with an only = [...] line in my config, the --only command line flag is ignored.

[GideonBear]

Completely unrelated: with an only = [...] line in my config, the --only command line flag is ignored.

That should not be the case. It should compound, e.g. only = [ a ] and --only b should result in steps a, b running.

[promisedlandt]

Completely unrelated: with an only = [...] line in my config, the --only command line flag is ignored.

That should not be the case. It should compound, e.g. only = [ a ] and --only b should result in steps a, b running.

Very possible that's what happened then. I had guix both in my config and the flag.
I would have expected the flag to override the config.