fix-pnpm

[kothavade]

Fixes #1018.

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
  • [x] I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the means to test it, please tag this person here.

[kothavade]

Tested to work with pnpm now, and if pnpm is not in path, npm runs as expected. pnpm's version is below the version that node deprecated -g, and it still uses -g (and doesn't support --location=global), making the specific check unnecessary.

[idanski]

Thanks! @vedkothavade