crypto/x509: skip known TestSystemVerify flakes on windows-*-2008 builders

The "-2008" builders are the only ones on which the failure has
been observed, so I suspect that it is due to a platform bug fixed in a
subsequent release.

Since no one has added a workaround since #19564 was filed over four
years ago, I'm assuming that no workaround is planned for this issue.
Let's add a skip for the known failure mode and call it at that.

Fixes #19564

Change-Id: Iefc22d1cc78bfdc79c845eb60cac22e26caf388c
Reviewed-on: https://go-review.googlesource.com/c/go/+/370377


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>