🔨 Fix: BIGTREE_E3_RRF doesn't use user RX/TX sizes

Fixes #22466. Regression from #22377.

ini/stm32f4.ini

 board               = marlin_STM32F407VGT6_CCM
 board_build.variant = MARLIN_BIGTREE_E3_RRF
 board_build.offset  = 0x8000
+extra_scripts       = ${common.extra_scripts}
 build_flags         = ${stm32_variant.build_flags}
                       -DSTM32F407_5VX
                       -DSERIAL_RX_BUFFER_SIZE=255