Remove stale symlink in `var/lib/pyenv` before renewing it

This is the fix for a problem in the `pyenv` formula found in #27629.

This removes stale symlink of `var/lib/pyenv/plugins/python-build` before renewing it. Without this fix, `FileUtils#ln_sf(src, dest)` will create nested symlink inside `dest` if the `dest` exists and is a directory.