Fix uv venv handling when VIRTUAL_ENV refers to an non-existant environment

[zanieb]

Closes #4072

This was an accidental change in #4029 in which I had updated the pull request to support virtual environments as requested in review and forgot to revert it.

Separately, we shouldn't fail if VIRTUAL_ENV points to an empty directory and SystemPython::Allowed is used, will address that separately.

[zanieb]

❯  VIRTUAL_ENV=/tmp/venv uv venv
  × failed to canonicalize path `/tmp/venv/bin/python3`
  ╰─▶ No such file or directory (os error 2)

❯ VIRTUAL_ENV=/tmp/venv cargo run -- venv -v
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/uv venv -v`
DEBUG Searching for Python interpreter in search path
DEBUG Found CPython 3.9.6 at `/usr/bin/python3` (search path)
Using Python 3.9.6 interpreter at: /Library/Developer/CommandLineTools/usr/bin/python3
Creating virtualenv at: .venv
INFO Removing existing directory
Activate with: source .venv/bin/activate