🐛 Fix Octopus build on case-sensitive FS (#22206)

ini/stm32f4.ini

 [env:BIGTREE_OCTOPUS_V1]
 platform          = ${common_stm32.platform}
 extends           = common_stm32
-board             = marlin_BigTree_octopus_v1
+board             = marlin_BigTree_Octopus_v1
 extra_scripts     = ${common.extra_scripts}
     pre:buildroot/share/PlatformIO/scripts/generic_create_variant.py
 build_flags       = ${common_stm32.build_flags}