go.mod: ignore cyclic dependency for tagging
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/mod,golang/mod