fix gnome shell extensions update object path

Standards checklist:

[x] The PR title is descriptive.
[x] The code compiles (cargo build)
[ ] The code passes rustfmt (cargo fmt)
[ ] 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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