test: disable flaky test/fixedbugs/issue10958.go

Updates #18589

Change-Id: I2c3bbc8257c68295051bd2e63e1e11794d0609c3
Reviewed-on: https://go-review.googlesource.com/40651
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue10958.go

-// +build !nacl
+// +build !nacl,disabled_see_issue_18589
 // buildrun -t 10  -gcflags=-d=ssa/insert_resched_checks/on,ssa/check/on
 
 // Copyright 2016 The Go Authors. All rights reserved.