add compat arg --user error message to pip install (#3424)
Resolves [#3419](https://github.com/astral-sh/uv/issues/3419)
## Summary
Add compatargs to pip install command and hint the user to create a venv
for --user arg.
## Test Plan
Tested it locally.
```bash
cargo run pip install --user flask
Compiling uv v0.1.39 (/home/ahmedilyas/uv/crates/uv)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.96s
Running `target/debug/uv pip install --user flask`
error: pip install's `--user` is unsupported (use a virtual environment instead).
```
