go.mod: ignore cyclic dependency for tagging

tools and mod have a cyclic dependency, which needs to be broken during
the upgrading/tagging process. This direction is due to the use of txtar
in the tests for the zip package, which is much less important than the
other direction.

For golang/go#48523.

Change-Id: Id5006280b0bc6b39f191b0281d542d251d80fff5
Reviewed-on: https://go-review.googlesource.com/c/mod/+/439335
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go.mod

@@ -4,5 +4,5 @@ go 1.17
 
 require (
 	golang.org/x/crypto v0.0.0-20210921155107-089bfa567519
-	golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e
+	golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e // tagx:ignore
 )