fix momentary layer bug when top layer is not &trans

Key release events released keys on the wrong layer if the 'top layer'
was not &trans above the &mo key.

base    <&mo 1>
layer 1 <&kp B>

This was caused by overwriting
`zmk_keymap_active_behavior_layer[position]` after the &mo key was
handled.

app/tests/momentary-layer/3-covered/native_posix.keymap

@@ -0,0 +1,33 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan-mock.h>
+
+/*
+this test verifies that the correct key is released when a layer is enabled "on top"
+and the original key is "covered".
+*/
+/ {
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&trans &mo 1
+				&trans &trans>;
+		};
+
+		layer_1 {
+			bindings = <
+				&trans &kp A
+				&trans &trans>;
+		};
+	};
+};
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,1,10) 
+		ZMK_MOCK_RELEASE(0,1,10)
+	>;
+};