zip: skip symlink test on Plan 9
zip: skip symlink test on Plan 9

Fixes golang/go#36041

Change-Id: I7b65ca053e994c56d6a354133daff81bcadd7220
Reviewed-on: https://go-review.googlesource.com/c/mod/+/210397
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/mod,golang/mod