fix: use env ZSH to compose oh-my-zsh install dir

[SteveLauC]

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

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Closes #426

cc @lime-desu who has already found this and posted it in discussions