release-test.yml: return of the Matrix notification

This time keep-it-simple-style with just a cURL command, using the
[API `m.room.message`](https://spec.matrix.org/latest/client-server-api/#events-2).

.github/workflows/release-test.yml

@@ -130,6 +130,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install lib32asan6
     - name: Run release tests
+      id: tests
       timeout-minutes: 350
       run: |
         RIOTBASE="$GITHUB_WORKSPACE/RIOT"
@@ -181,6 +182,25 @@ jobs:
         mkdir test-reports/
         junit2html ${REPORT_XML} ${REPORT_NAME}.html
         cp ${REPORT_XML} ${REPORT_NAME}.xml
+    - name: Generate result message
+      if: always()
+      id: generate_results
+      run: |
+        if [ "${{ steps.tests.conclusion }}" == "success" ]; then
+          nice_str="&#x2705; <strong>PASSED</strong>"
+        elif [ "${{ steps.tests.conclusion }}" == "failure" ]; then
+          nice_str="&#x274C; <strong>FAILED</strong>"
+        fi
+        echo "nice_str=${nice_str}" >> ${GITHUB_OUTPUT}
+    - name: Report to Matrix channel
+      if: ${{ always() && steps.generate_results.outputs.nice_str != '' }}
+      run: |
+        MESSAGE="${{ steps.generate_results.outputs.nice_str}}: Release tests" \
+          "[${{ matrix.pytest_mark }}, ${{ matrix.sudo }}] on '<em>${{ github.event_name }}</em>':" \
+          "${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"; \
+        curl -H "Content-Type: application/json" -X POST \
+          --data '{"msgtype":"m.text","format":"org.matrix.custom.html","body":"${MESSAGE}","formatted_body":"${MESSAGE}"}' \
+          "https://matrix.org/_matrix/client/v3/rooms/${{env.RIOT_CI_RELEASE_REPORT_CHANNEL}}/send/m.room.message?access_token=${{secrets.MATRIX_RIOT_CI_ACCESS_TOKEN}}"
     - uses: actions/upload-artifact@v2
       if: always()
       with: