sftp: set DirModTimeUpdatesOnWrite to fix integration tests

backend/sftp/sftp.go

@@ -1066,9 +1066,10 @@ func NewFsWithConnection(ctx context.Context, f *Fs, name string, root string, m
 	}
 
 	f.features = (&fs.Features{
-		CanHaveEmptyDirectories: true,
-		SlowHash:                true,
-		PartialUploads:          true,
+		CanHaveEmptyDirectories:  true,
+		SlowHash:                 true,
+		PartialUploads:           true,
+		DirModTimeUpdatesOnWrite: true, // indicate writing files to a directory updates its modtime
 	}).Fill(ctx, f)
 	if !opt.CopyIsHardlink {
 		// Disable server side copy unless --sftp-copy-is-hardlink is set