Upgrade test: an older offer is nothing to upgrade to either, and show a stuck reboot by itself

The early stop compared the offered version with the installed one for
equality. Started from a release newer than the channel holds, the test
went on to the updater with nothing to install. It now asks pacman's
vercmp and stops when the offer is the same or older.

The picture of a reboot that stuck was inside the condition that wants
both the before and the after picture, so it was left out exactly when
there was no after to show. It now has its own condition.

.github/workflows/upgrade-test.yml

@@ -200,12 +200,14 @@ jobs:
               echo "| Before, $FROM | After, on $TO |"
               echo "| --- | --- |"
               echo "| ![before]($PICTURES/before.png) | ![after]($PICTURES/after.png) |"
-              if [ -s evidence/stuck.png ]; then
-                echo
-                echo "The reboot did not come back by itself. This is what the screen showed:"
-                echo
-                echo "![stuck]($PICTURES/stuck.png)"
-              fi
+            fi
+            # The picture of a reboot that stuck stands by itself: it is the
+            # one worth having when there is no "after" to show.
+            if [ -n "$PICTURES" ] && [ -s evidence/stuck.png ]; then
+              echo
+              echo "The reboot did not come back by itself. This is what the screen showed:"
+              echo
+              echo "![stuck]($PICTURES/stuck.png)"
             fi
             echo
             echo "[Run $GITHUB_RUN_ID]($GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID), attempt $GITHUB_RUN_ATTEMPT, at \`${GITHUB_SHA:0:7}\`."