version: use ProxyFromEnvironment

Thanks to Benjamin Bytheway for pointing out its omission.

Change-Id: Iee53e3d77fe575d1bbde7f3bc0c4693d293857a2
Reviewed-on: https://go-review.googlesource.com/35845
Reviewed-by: Chris Broadfoot <cbro@golang.org>

version/version.go

@@ -315,6 +315,7 @@ func copyFromURL(dstFile, srcURL string) (err error) {
 			// (Not that GCS would try to compress it, though)
 			DisableCompression: true,
 			DisableKeepAlives:  true,
+			Proxy:              http.ProxyFromEnvironment,
 		}},
 	}
 	res, err := c.Get(srcURL)