try

.github/workflows/notify.yaml

@@ -1,5 +1,6 @@
 name: notify
-
+env:
+  icons: '{"success" : "white_check_mark", "failure": "white_check_mark", "canceled": "", "start": ""}'
 on:
   workflow_call:
     inputs:
@@ -15,6 +16,6 @@ jobs:
           MATTERMOST_WEBHOOK_URL: ${{ secrets.MM_WEBHOOK_URL }}
           PAYLOAD: |
             { 
-              "text": "${{ github.repository }} [build](${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_number }}) was ${{ inputs.result }} :white_check_mark:" 
+              "text": "${{ github.repository }} [${{ github.workflow }}](${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_number }}) was ${{ inputs.result }} ${{ fromJson(env.icons)[inputs.result]}}" 
             }