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 <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>

Author: Bryan C. Mills

src/crypto/x509/verify_test.go

@@ -13,6 +13,7 @@ import (
 	"encoding/pem"
 	"errors"
 	"fmt"
+	"internal/testenv"
 	"math/big"
 	"runtime"
 	"strings"
@@ -469,6 +470,9 @@ func testVerify(t *testing.T, test verifyTest, useSystemRoots bool) {
 	chains, err := leaf.Verify(opts)
 
 	if test.errorCallback == nil && err != nil {
+		if runtime.GOOS == "windows" && strings.HasSuffix(testenv.Builder(), "-2008") && err.Error() == "x509: certificate signed by unknown authority" {
+			testenv.SkipFlaky(t, 19564)
+		}
 		t.Fatalf("unexpected error: %v", err)
 	}
 	if test.errorCallback != nil {