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_windows.go

@@ -3,6 +3,7 @@
 package quic
 
 import (
+	"net"
 	"syscall"
 
 	"golang.org/x/sys/windows"
@@ -34,4 +35,8 @@ func inspectWriteBuffer(c syscall.RawConn) (int, error) {
 	return size, serr
 }
 
+type packetInfo struct {
+	addr net.IP
+}
+
 func (i *packetInfo) OOB() []byte { return nil }