derp,wgengine/magicsock: don't assume stringer is in $PATH for go:generate

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_server.go

@@ -737,7 +737,7 @@ func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 // dropReason is why we dropped a DERP frame.
 type dropReason int
 
-//go:generate go run tailscale.com/cmd/addlicense -year 2021 -file dropreason_string.go stringer -type=dropReason -trimprefix=dropReason
+//go:generate go run tailscale.com/cmd/addlicense -year 2021 -file dropreason_string.go go run golang.org/x/tools/cmd/stringer -type=dropReason -trimprefix=dropReason
 
 const (
 	dropReasonUnknownDest      dropReason = iota // unknown destination pubkey

wgengine/magicsock/magicsock.go

@@ -3490,7 +3490,7 @@ func (de *discoEndpoint) sendDiscoPing(ep netaddr.IPPort, txid stun.TxID, logLev
 // discoPingPurpose is the reason why a discovery ping message was sent.
 type discoPingPurpose int
 
-//go:generate go run tailscale.com/cmd/addlicense -year 2020 -file discopingpurpose_string.go stringer -type=discoPingPurpose -trimprefix=ping
+//go:generate go run tailscale.com/cmd/addlicense -year 2020 -file discopingpurpose_string.go go run golang.org/x/tools/cmd/stringer -type=discoPingPurpose -trimprefix=ping
 const (
 	// pingDiscovery means that purpose of a ping was to see if a
 	// path was valid.