Fix $GOPATH mounting on Windows for local builds

[kevinpfab]

Resolves #115

The EXT_GOPATH value used inside the xgo docker build script requires Linux path separators. If xgo was used on Windows and building a local build, the EXT_GOPATH argument would forward along a Windows-style path.

This patch forwards along the path with the proper Linux path separators regardless of the os.PathSeparator value.

Before: EXT_GOPATH=\ext-go\1
After: EXT_GOPATH=/ext-go/1

[luiscarrasco]

This fixes getting the errors mentioned in #115 on my windows setup.

[jicg]

me too。 thanks

[thinkgos]

me too, got it ,thanks!!