Fix YAML parsing in the upstream sync workflow

.github/workflows/sync-from-accelerate.yml

@@ -113,29 +113,29 @@ jobs:
         env:
           GH_TOKEN: ${{ github.token }}
         run: |
-          BODY=$(cat <<EOF
-This pull request synchronizes DIDAS with the canonical Accelerate upstream.
-
-Upstream: https://github.com/$UPSTREAM_REPOSITORY
-Upstream commit: `$UPSTREAM_SHA`
-
-Repository-specific DIDAS publication and synchronization workflows are preserved.
-The synchronized tree passed the repository's validation commands before this PR was created.
-
-This PR is generated by GitHub Actions.
-EOF
-          )
+          BODY_FILE="$RUNNER_TEMP/sync-pr-body.md"
+          printf '%s\n' \
+            'This pull request synchronizes DIDAS with the canonical Accelerate upstream.' \
+            '' \
+            "Upstream: https://github.com/$UPSTREAM_REPOSITORY" \
+            "Upstream commit: \`$UPSTREAM_SHA\`" \
+            '' \
+            'Repository-specific DIDAS publication and synchronization workflows are preserved.' \
+            "The synchronized tree passed the repository's validation commands before this PR was created." \
+            '' \
+            'This PR is generated by GitHub Actions.' \
+            > "$BODY_FILE"
 
           if NUMBER=$(gh pr view "$SYNC_BRANCH" --json number --jq .number 2>/dev/null); then
             gh pr edit "$NUMBER" \
               --title "Sync from Accelerate upstream" \
-              --body "$BODY"
+              --body-file "$BODY_FILE"
           else
             gh pr create \
               --base main \
               --head "$SYNC_BRANCH" \
               --title "Sync from Accelerate upstream" \
-              --body "$BODY"
+              --body-file "$BODY_FILE"
           fi
 
       - name: Close stale synchronization PR when no drift remains