[STYLE] Improved quoting in CI-MATs.yml (- WIP #266 -)

Changes in file .github/workflows/CI-MATs.yml:
 * refactored `$GITHUB_OUTPUT` to `"$GITHUB_OUTPUT"`

Author: reactive-firewall

.github/workflows/CI-MATs.yml

@@ -43,17 +43,17 @@ jobs:
       - id: check
         run: |
           if [[ "${{ github.event.workflow_run.conclusion }}" == "success" ]]; then
-            echo "should_run=true" >> $GITHUB_OUTPUT
+            echo "should_run=true" >> "$GITHUB_OUTPUT"
           else
-            echo "should_run=false" >> $GITHUB_OUTPUT
+            echo "should_run=false" >> "$GITHUB_OUTPUT"
           fi
       - id: get_env
         run: |
           ENV_VALUE=$(gh api "${{ github.event.workflow_run.artifacts_url }}" --jq '.environment')
           if [[ -n "$ENV_VALUE" ]]; then
-            echo "environment=$ENV_VALUE" >> $GITHUB_OUTPUT
+            echo "environment=$ENV_VALUE" >> "$GITHUB_OUTPUT"
           else
-            echo "environment=Experimenting" >> $GITHUB_OUTPUT  # Default fallback
+            echo "environment=Experimenting" >> "$GITHUB_OUTPUT"  # Default fallback
           fi
 
   MATS:
@@ -121,7 +121,7 @@ jobs:
       - id: check_status
         run: |
           if [[ "${{ needs.MATS.result }}" == "success" ]]; then
-            echo "mats_success=true" >> $GITHUB_OUTPUT
+            echo "mats_success=true" >> "$GITHUB_OUTPUT"
           else
-            echo "mats_success=false" >> $GITHUB_OUTPUT
+            echo "mats_success=false" >> "$GITHUB_OUTPUT"
           fi