chore: save plan outcome to an intermediate variable [skip ci]

.github/workflows/diff-cdktf-stacks.yml

@@ -47,6 +47,9 @@ jobs:
           terraform plan -no-color | tee ../../../plan_stdout_${{ matrix.stack }}.txt; echo $?
           set +o pipefail
         continue-on-error: true
+      - name: Save results into a variable
+        id: plan_outcome
+        run: echo "value=${{steps.plan.outcome}}" >> $GITHUB_OUTPUT
       - name: Comment the plan output on the PR
         if: contains(github.event_name, 'pull_request') # pull_request or pull_request_target
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
@@ -66,7 +69,7 @@ jobs:
               stackName: "${{matrix.stack}}" 
             })
       - name: Fail this check if the plan was not successful
-        if: ${{ steps.plan.outcome }} != 'success'
+        if: ${{ steps.plan_outcome.outputs.value }} != 'success'
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
         with:
           script: |