Fix invalid timestamp of the "public" folder

Special thanks to both Richard Mortimer (@oldelvet) and
Joshua M. Clulow (@jclulow) for their analysis and suggested fix:

 * https://github.com/gohugoio/hugo/issues/6161#issuecomment-574336088
 * https://github.com/gohugoio/hugo/issues/6161#issuecomment-596805273

Fixes #6161

hugofs/fileinfo.go

@@ -280,7 +280,7 @@ func (fi *dirNameOnlyFileInfo) Mode() os.FileMode {
 }
 
 func (fi *dirNameOnlyFileInfo) ModTime() time.Time {
-	return time.Time{}
+	return time.Now()
 }
 
 func (fi *dirNameOnlyFileInfo) IsDir() bool {