Fix build script always using python3 in OSX builds

[althonos]

Hi!

I noticed that when compiling pyo3 on OSX for a non-standard Python interpreter (such as PyPy), the build script would at some point assume python or python3 to be the name of the interpreter, despite PYTHON_SYS_EXECUTABLE being set. This was caused by get_macos_linkmodel using the default PYTHON_INTERPRETER eventually.

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• Run cargo fmt (This is checked by travis ci) ✅
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[kngwyu]

the build script would at some point assume python or python3 to be the name of the interpreter, despite PYTHON_SYS_EXECUTABLE being set.

Ah, it's because of #596.
Sorry, I overlooked the problem when reviewing, and thank you for fixing it!