Search for python3 in unix virtual environments - #3798

Unix environment discovery prefers `bin/python3` even when `bin/python`
exists. This differs from the interpreter exposed by an activated
environment and from pip's handling of directory-valued `--python`
arguments.

Prefer `bin/python`, while retaining `bin/python3` as a fallback for
installations that omit the unversioned executable. This reverses the
precedence introduced in #3798 without removing support for
`python3`-only layouts. Explicit executable paths and version-specific
`PATH` discovery are unchanged.

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Co-authored-by: Zanie Blue <contact@zanie.dev>