Fix variable for build fornat

Author: rffontenelle

.github/workflows/check.yml

@@ -81,7 +81,7 @@ jobs:
         if: always() && steps.build.outcome == 'failure' && matrix.format == 'html'
         id: prepare
         run: |
-          scripts/prepmsg.sh logs/sphinxwarnings-${format}.txt logs/notify.txt
+          scripts/prepmsg.sh logs/sphinxwarnings-${{ matrix.format }}.txt logs/notify.txt
           cat logs/notify.txt
         env:
           GITHUB_JOB: ${{ github.job }}