fix(pnpm): install don't take a list of packages

[KuSh]

Summary

Remove packages handling for pnpm install command.

While testing another PR, I stumbled across this error:

$ cargo run -- pnpm install -h
thread 'main' (239705) panicked at ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/clap_builder-4.5.60/src/builder/debug_asserts.rs:570:9:
Positional argument `[ARGS]...` *must* have `required(true)` or `last(true)` set because a prior positional argument (`[PACKAGES]...`) has `num_args(1..)`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Upon checking pnpm documentation, it turns out that pnpm install doesn't take a list of packages.

Test plan

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