ci(gh-actions): Set the PR description to be the same as the commit msg

.github/workflows/update-flake-lock.yml

@@ -36,7 +36,7 @@ jobs:
         with:
           token: ${{ steps.generate-token.outputs.token }}
           title: 'Update Nix Flake lockfile'
-          commit-message: ${{ env.COMMIT_MSG }}
+          body-path: ./commit_msg_body.txt
           branch: 'create-pull-request/update-flake-lockfile'
           delete-branch: true
           branch-suffix: timestamp

scripts/commit_flake_update.bash

@@ -21,8 +21,19 @@ if [[ "$commit_after_update" = "$current_commit" ]]; then
   exit 0
 fi
 
+msg_file=./commit_msg_body.txt
+{
+  echo '```'
+  git log -1 '--pretty=format:%b' | sed '1,2d'
+  echo '```'
+} > $msg_file
+
 git commit --amend -F - <<EOF
 chore(flake.lock): Update all Flake inputs ($(date -I))
 
-$(git log -1 '--pretty=format:%b' | sed '1,2d')
+$(cat $msg_file)
 EOF
+
+if [ -z "${CI+x}" ]; then
+  rm -v $msg_file
+fi