github: fix payload in check-pebble-dep.yml

Epic: none
Release note: None

Author: RaduBerinde

.github/workflows/check-pebble-dep.yml

@@ -31,13 +31,14 @@ jobs:
         if: steps.run_script.outputs.exitcode != '0'
         uses: slackapi/[email protected]
         with:
+          errors: true
           method: chat.postMessage
           token: ${{ secrets.PEBBLE_SLACK_BOT_TOKEN }}
           # The channel ID is for #storage-notifications.
           payload: |
-            channel: C08JE13CM9S
-            text: |
-            Some Pebble dependencies are not up to date. Details below:
-            ```
-            ${{ steps.run_script.outputs.output }}
-            ```
+            {
+              "channel": "C08JE13CM9S",
+              "text": ${{ toJson(format(
+                'Some Pebble dependencies are not up to date. Details below:\n```{0}```',
+                steps.run_script.outputs.output)) }}
+            }