gh-143768: Replace a dangling interpreter symlink when creating a venv

[claydugo]

A dangling interpreter symlink in the target directory caused venv to crash with --copies and to silently leave the broken link in place with symlinks.
symlink_or_copy now unlinks a dangling destination symlink first, matching how a stale regular file is already replaced on re-run.

Supersedes #143770, which got stuck in a wedged change-detection state after a bad merge via the GitHub UI.

• Issue: venv fails with confusing ENOENT when existing venv has broken symlinks #143768

[brettcannon]

I just realized that there's no doc update about the semantic change. Otherwise LGTM!

[bedevere-app]

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