.github/workflows: avoid substitution wonkiness from step output.

Author: kivikakk

.github/workflows/release.yml

@@ -60,7 +60,8 @@ jobs:
             - name: Update changelog.txt
               run: |
                   echo "# [v${{ steps.version-label.outputs.version }}] - `date +%Y-%m-%d`" >> changelog.txt.tmp
-                  echo "${{steps.generate-release-notes.outputs.changelog}}" >> changelog.txt.tmp
+                  echo '' > changelog.txt
+                  echo '${{steps.generate-release-notes.outputs.changelog}}' >> changelog.txt.tmp
                   echo '' >> changelog.txt
                   cat changelog.txt >> changelog.txt.tmp
                   mv changelog.txt.tmp changelog.txt