Fix mega2560ext environment (#19730)

platformio.ini

@@ -445,7 +445,7 @@ board               = megaatmega2560
 #
 [env:mega2560ext]
 platform            = atmelavr
-extends             = mega2560
+extends             = env:mega2560
 board               = megaatmega2560
 board_build.variant = megaextendedpins
 extra_scripts       = ${common.extra_scripts}