gh-99204: Calculate base_executable by alternate names in POSIX venvs (GH-99206)

Check to see if `base_executable` exists. If it does not, attempt
to use known alternative names of the python binary to find an
executable in the path specified by `home`.

If no alternative is found, previous behavior is preserved.

Signed-off-by: Vincent Fazio <vfazio@gmail.com>

Signed-off-by: Vincent Fazio <vfazio@gmail.com>