Set up DEFAULT_FOLDER for primekb/meridian (#20367)

keyboards/primekb/meridian/rules.mk

@@ -13,3 +13,5 @@ AUDIO_ENABLE = no           # Audio output
 
 # Enter lower-power sleep mode when on the ChibiOS idle thread
 OPT_DEFS += -DCORTEX_ENABLE_WFI_IDLE=TRUE
+
+DEFAULT_FOLDER = primekb/meridian/ktr1010