src: env.cc should not include unistd.h on Windows

[mscdex]

unistd.h is being included unconditionally in src/env.cc since 94b9948, but Windows does not have this header.

/cc @JungMinu

[targos]

Example of failure: https://ci.nodejs.org/job/node-compile-windows/480/label=win-vs2013/console
src\env.cc(4): fatal error C1083: Cannot open include file: 'unistd.h': No such file or directory [c:\workspace\node-compile-windows\label\win-vs2013\node.vcxproj]