[PIO] Switch to core_stage for core 2.6.0 builds

To be able to use this merged PR: esp8266/Arduino#6484

Author: TD-er

platformio.ini

@@ -245,8 +245,8 @@ platform                  = ${core_2_5_2.platform}
 
 [beta_platform]
 build_unflags             =
-build_flags               = ${core_2_6_0.build_flags} ${common.build_flags} -DARDUINO_ESP8266_RELEASE='"2.6.0-dev"'
-platform                  = ${core_2_6_0.platform}
+build_flags               = ${core_stage.build_flags} ${common.build_flags} -DARDUINO_ESP8266_RELEASE='"2.6.0-dev"'
+platform                  = ${core_stage.platform}