Also save elf file (debugging info)

Author: henrygab

.github/workflows/build.yml

@@ -43,4 +43,6 @@ jobs:
       - uses: actions/upload-artifact@v2
         with:
           name: firmware
-          path: .pio/build/*/firmware.bin
+          path: |
+            .pio/build/*/firmware.bin
+            .pio/build/*/firmware.elf