Replace executables with broken symlinks during uv python install

[zanieb]

I somehow got in a state where we'd fail to install with

error: Failed to install cpython-3.13.0-macos-aarch64-none
  Caused by: Executable already exists at `/Users/zb/.local/bin/python3` but is not managed by uv; use `--force` to replace it
error: Failed to install cpython-3.13.0-macos-aarch64-none
  Caused by: Executable already exists at `/Users/zb/.local/bin/python` but is not managed by uv; use `--force` to replace it

but python / python3 were managed by uv, they just were linked to an installation that was deleted.

This updates the logic to replace broken executables that are broken symlinks. We apply this to broken links regardless of whether or not we think the target is managed by uv.

[charliermarsh]

This logic only runs on Linux, right? It doesn't look like it's gated, but I assume the error it's handling only runs on Linux?

[zanieb]

I think this technically would run on Windows too (which is valid, you could have a broken symlink it's just rare).