runtime/race: use race build tag on syso_test.go

All other test files in the runtime/race package have race build
tag, except syso_test.go. The test is only relevant if the race
detector is supported. So apply the build tag.

Fixes #46931.

Change-Id: Icdb94214d3821b4ccf61133412ef39b4d7cc7691
Reviewed-on: https://go-review.googlesource.com/c/go/+/331050


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Elias Naur <mail@eliasnaur.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>