Workflow: Fix branch name

Author: nseidle

.github/workflows/compile-rtk-firmware.yml

@@ -125,7 +125,7 @@ jobs:
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           directory: ./Firmware/RTK_Surveyor
-          branch: $BRANCH
+          branch: ${{ env.BRANCH }}
 
       - name: Compile Sketch
         run: arduino-cli compile --fqbn "esp32:esp32:esp32":DebugLevel=${{ env.DEBUG_LEVEL }} ./Firmware/RTK_Surveyor/RTK_Surveyor.ino