contrib: don't keep PREFIX/bin in the PATH

And other PREFIX/bin related patches.

PREFIX/bin contains cross-compiled binaries. It may be binaries native or runnable to the local machine, but they are not supposed to be used, as they are not used when cross-compiling. binaries we want to use are in BUILDPREFIX/bin aka BUILDBINDIR. And that one is always added to the PATH in contribs.

Even worse, it had priority over the regular PATH so if you have a native tool and a cross-compiled one, the cross-compiled one would take priority.