add stringcar_m0_express to boards list

Author: CedarGroveStudios

.github/workflows/build.yml

@@ -147,6 +147,7 @@ jobs:
         - "sparkfun_samd21_mini"
         - "stm32f411ve_discovery"
         - "stm32f412zg_discovery"
+        - "stringcar_m0_express"
         - "trellis_m4_express"
         - "trinket_m0"
         - "trinket_m0_haxpress"