fix(pip.install.wheel): support variations on `pythonw.exe` (#5259)
Emulate `pip`'s behaviour and find `pythonw` executable by doing an
`s/python/pythonw/g` style transformation, as opposed to assuming a
constant `pythonw.exe` path.
See #5256 for more detail e.g. why this is a useful behaviour to
emulate.
Fixes: #5256
