fix(pnpm): list command not working

[KuSh]

Summary

rtk pnpm list fails to parse the json output:

❯ rtk -vvv pnpm list
[RTK:DEGRADED] pnpm list parser: JSON parse failed: invalid type: sequence, expected struct PnpmListOutput at line 1 column 0
pkg:16 ^0

Output format struct doesn't seem to have changed during the last 7 years

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk <command> output inspected

❯ ../rtk/target/debug/rtk -vvv pnpm list
pnpm list (Tier 1: Full JSON parse)
pkg:51 ^0

[FlorianBruniaux]

LGTM — clean fix, tests pass.