.github/workflows: Build ev3rt uImage only.

Since we don't package a u-boot file yet, don't try to build the bootable firmware.

Author: laurensvalk

.github/workflows/build.yml

@@ -282,7 +282,7 @@ jobs:
     - name: Fix file permission
       run: chmod +x micropython/mpy-cross/build/mpy-cross
     - name: Build
-      run: make $MAKEOPTS -C bricks/ev3rt
+      run: make $MAKEOPTS -C bricks/ev3rt uImage
 
   debug:
     name: debug firmware