fixup!(ci): Run create-dmg until it succeeds

This properly stores the result of the increment operation in a
variable, where previously that evaluation could return 1 and mess up
with the exit status of this build step.

That would happen when create-dmg failed in the first try, but succeeded
in the next one.

I've also added "set -x" to enable Bash's command tracing, making it
easier to spot how many times create-dmg gets invoked.

.github/workflows/macos.yml

@@ -186,6 +186,7 @@ jobs:
 
       - name: Build dmg (${{ matrix.build-type }})
         run: |
+          set -x
           cd build
           # FIXME: Undo this overengineered crap once the following issue gets figured out:
           #        https://github.com/actions/runner-images/issues/7522
@@ -211,7 +212,7 @@ jobs:
                   echo 'Error: create-dmg did not succeed even after 10 tries.'
                   exit 1
               fi
-              ((i++))
+              i=$((i+1))
           done
 
       - name: Notarize