cmd/go/internal/cache: verify that timestamp is positive

An apparent typo caused us to re-check size instead.

Updates #29667

Change-Id: Icc1fb9cef1ba77b8490c8fbe6c343be06ff48d8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/188020
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -173,7 +173,7 @@ func (c *Cache) get(id ActionID) (Entry, error) {
 		i++
 	}
 	tm, err := strconv.ParseInt(string(etime[i:]), 10, 64)
-	if err != nil || size < 0 {
+	if err != nil || tm < 0 {
 		return missing()
 	}