cmd/go: always copy files on Windows

Copying ensures that we respect the NTFS permissions of the parent folder.
I don't know if there is a way to tell when it is safe to simply rename.

Fixes #22343

Change-Id: I424bfe655b53b0e0fe425ce92bbc15450d52d851
Reviewed-on: https://go-review.googlesource.com/72910
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/cmd/go/internal/work/exec.go

@@ -18,6 +18,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -1065,6 +1066,14 @@ func (b *Builder) moveOrCopyFile(a *Action, dst, src string, perm os.FileMode, f
 		return b.copyFile(a, dst, src, perm, force)
 	}
 
+	// On Windows, always copy the file, so that we respect the NTFS
+	// permissions of the parent folder. https://golang.org/issue/22343.
+	// What matters here is not cfg.Goos (the system we are building
+	// for) but runtime.GOOS (the system we are building on).
+	if runtime.GOOS == "windows" {
+		return b.copyFile(a, dst, src, perm, force)
+	}
+
 	// If the destination directory has the group sticky bit set,
 	// we have to copy the file to retain the correct permissions.
 	// https://golang.org/issue/18878