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_no_oob.go

@@ -11,4 +11,8 @@ func newConn(c net.PacketConn, supportsDF bool) (*basicConn, error) {
 func inspectReadBuffer(any) (int, error)  { return 0, nil }
 func inspectWriteBuffer(any) (int, error) { return 0, nil }
 
+type packetInfo struct {
+	addr net.IP
+}
+
 func (i *packetInfo) OOB() []byte { return nil }