ci: run linter on all supported platforms (#3816)

* only define packetInfo.ifIndex on platforms where it's actually used

* fix comment and stylecheck for IP_DONTFRAGMENT on Windows

* fix build flags on test file

* ci: run golangci-lint on multiple platforms

sys_conn_oob.go

@@ -264,6 +264,11 @@ func (c *oobConn) capabilities() connCapabilities {
 	return c.cap
 }
 
+type packetInfo struct {
+	addr    net.IP
+	ifIndex uint32
+}
+
 func (info *packetInfo) OOB() []byte {
 	if info == nil {
 		return nil