Revert "build: use %PYTHON% instead of python"

This reverts commit 91943a99.

Old commit cherry-picked in but found to cause problems with .msi
creation on Windows.
Original change is mostly pointless because V8 hard-wires
`python` anyway.

PR-URL: https://github.com/iojs/io.js/pull/1475


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>