The previous commit removed the !race build constraint, intending for
the test to skip gracefully when qemu isn't installed. However, the
race builder in CI doesn't have qemu installed (to save time), so the
test fails there.
Instead of using a build constraint, check racebuild.On at runtime to
allow graceful skipping specifically on the race builder while still
requiring qemu on other CI builders.
Change-Id: I6690e2cd313b297240ecfaaf3439c4a2c24b33dd
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>