ci: one MCPP_SOURCE_REF definition, not two

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -12,7 +12,6 @@ on:
         default: ""
 env:
   MCPP_SOURCE_REF: ${{ github.event.inputs.mcpp_ref || vars.MCPP_SOURCE_REF }}
-  MCPP_SOURCE_REF: ${{ github.event.inputs.mcpp_ref || vars.MCPP_SOURCE_REF }}
 
 jobs:
   build: