Skip deno upgrade unless installed under .deno

[tranzystorekk]

Standards checklist:

• The PR title is descriptive.
• 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
  • I also tested that Topgrade skips the step where needed

Deno can be installed from cargo or arch linux's repositories, so upgrade should only be called for user-wide shell-script installations.