cmd/go: use Windows %AppData% for build cache if %LocalAppData% is empty

Fixes #23190

Change-Id: I96805aaab44ddaae6098b4e3af30f9e52585eae0
Reviewed-on: https://go-review.googlesource.com/87675
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/cache/default.go

@@ -68,6 +68,11 @@ func DefaultDir() string {
 	switch runtime.GOOS {
 	case "windows":
 		dir = os.Getenv("LocalAppData")
+		if dir == "" {
+			// Fall back to %AppData%, the old name of
+			// %LocalAppData% on Windows XP.
+			dir = os.Getenv("AppData")
+		}
 		if dir == "" {
 			return "off"
 		}