Get build script to work on Windows (Cygwin)

Two major changes needed:

* Instead of symlinking code, copy code to a tmp dir for build. Go compiler doesn’t work with Windows symlink and Cygwin’s simulated symlink.
* Use `cygpath` to convert *nix path to Windows path for pants in GOPATH. Go compiler doesn’t work with Cygwin path for the GOPATH environment variable on Windows - it requires a Windows path.

As part of the changes, the code to detect freshness of the build and skip compilation is removed since:

* Go compiler is fast and will only get faster in the future. It doesn’t seem necessary to complicate the code with such detection.
* It may be surprising the build script actually skip compilation for some cases.
* It’s more convenient to test a fresh build when debugging the build script.