[9front] upas/vf: install %τ format specifier

upas/vf was converted to use tmdate, but the formatter was never
installed.  This caused it to send attachments to validateattachment
with header `From virusfilter %τ%`, which always failed since upas/fs
would just skip over the message.

sys/src/cmd/upas/vf/vf.c

@@ -140,6 +140,8 @@ main(int argc, char **argv)
 	if(argc)
 		usage();
 
+	tmfmtinstall();
+
 	Binit(&in, 0, OREAD);
 	Binit(&out, 1, OWRITE);