fix: bug with matrix job (#560)

.github/workflows/manual-matrix-test.yml

@@ -8,7 +8,7 @@ jobs:
     name: Get changes
     runs-on: ubuntu-latest
     outputs:
-      matrix: "{\"container\": ${{ steps.changed-files.outputs.all_changed_files }} }"
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -21,7 +21,10 @@ jobs:
           json: true
       - name: List all changed files
         run: |
-          echo '${{ toJSON(steps.changed-files.outputs.all_changed_files) }}'
+          echo '${{ steps.changed-files.outputs.all_changed_files }}'
+
+      - id: set-matrix
+        run: echo "::set-output name=matrix::{\"container\":${{ steps.changed-files.outputs.all_changed_files }}}"
 
   matrix-job:
     name: Run Matrix Job