cmd/tailscaled: don't require root for --cleanup

Without this, the integration tests fail locally for me:

--- FAIL: TestCollectPanic (7.61s)
    integration.go:74: built [tailscale.com/cmd/tailscaled tailscale.com/cmd/tailscale] in 1.59s
    integration_test.go:102: initial run: tailscaled requires root; use sudo tailscaled (or use --tun=userspace-networking)
    integration_test.go:108: cleanup failed: exit status 1: "tailscaled requires root; use sudo tailscaled (or use --tun=userspace-networking)\n"
    stuntest.go:64: STUN server shutdown
FAIL
FAIL	tailscale.com/tstest/integration	9.678s
FAIL

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f