Show output on terraform plan failure

When I run the workflow using terraform v1.0.7 and have a failed plan, no output will be shown in the commit message. I have solved this by adding the `steps.plan.outputs.stderr` to the PLAN env variable to get this to print out. Perhaps I am doing something incorrectly that I need to do this workaround? Any reason this shouldn't be included in the working example?

Before:
```
env:
PLAN: "terraform\n${{ steps.plan.outputs.stdout }}"
```
After:
```
env:
PLAN: "terraform\n${{ steps.plan.outputs.stdout }}"\n${{ steps.plan.outputs.stderr }}"
```