Deprecate --system and --no-system in uv venv

[zanieb]

e.g.

❯ cargo run -- venv --no-system
    Blocking waiting for file lock on build directory
   Compiling uv v0.2.34 (/Users/zb/workspace/uv/crates/uv)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.85s
     Running `target/debug/uv venv --no-system`
warning: The `--no-system` flag has no effect, a system Python interpreter is always used in `uv venv`
Using Python 3.12.4 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate

❯ cargo run -- venv --system
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv venv --system`
warning: The `--system` flag has no effect, a system Python interpreter is always used in `uv venv`
Using Python 3.12.4 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate