find_program: use Meson's Python3 for non-executable Python scripts

Whenever a non-executable Python script is found by find_program, currently
Haiku and Windows replace a python3 from the shebang line with the one that
was used by Meson.  Extend this behavior to POSIX systems so that it is
easy to test a program with multiple Python versions.

Currently this is particularly important for generators, because
they don't allow files in the arguments and thus you cannot do
something like

   g = generator(pymod.find_installation(), ...,
                 arguments: [files('myscript.py'), ...])

With this patch, instead, you can just do

   g = generator(find_program('myscript.py'), ...)

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