Keep the native prefix in front of PATH

[isuruf]

cc @radarhere

[radarhere]

While I appreciate the thought (trying to solve a problem I was having in #495), this doesn't resolve the issue for me.

Trying to run cjpeg after a universal2 build fails by default.
Using this change, it still fails.
Swapping the order of the builds works.

[isuruf]

@radarhere, try the new commit.

[radarhere]

Hey, that does work! Thanks very much

[mattip]

Will there be cases this breaks something? I am not sure what the implications are of changing the $PATH.

[radarhere]

In case it is relevant, the second commit is what fixed my situation, independently of the first.

[mattip]

#502 is merged. Do we still need this one?

[radarhere]

Not on my account. The remaining change here would actually introduce problems to pillow-wheels.

[isuruf]

How would it introduce problems?

[radarhere]

I've tested further and it was only a problem where a path has changed when installing xcb - something that could probably be worked around.

By no means did I want to object to this for minor reasons though. If this makes it in on its own merits, that's fine. Just don't do it to solve any probem that I had raised.

[mattip]

Whoops, parts of this were done in #531. Do we need the other changes for cross-compiling on macos?