add kiss radio env to most of the variants

variants/xiao_c6/platformio.ini

@@ -184,3 +184,8 @@ build_src_filter = ${WHY2025_badge.build_src_filter}
 lib_deps =
   ${WHY2025_badge.lib_deps}
   densaugeo/base64 @ ~1.4.0
+
+[env:Xiao_C6_kiss_modem]
+extends = Xiao_C6
+build_src_filter = ${Xiao_C6.build_src_filter}
+  +<../examples/kiss_modem/>