ref: add one more backend test shard (#45641)

kick the can down the road




<!-- Describe your PR here. -->

.github/workflows/backend.yml

@@ -74,12 +74,12 @@ jobs:
       fail-fast: false
       matrix:
         # XXX: When updating this, make sure you also update MATRIX_INSTANCE_TOTAL.
-        instance: [0, 1, 2, 3]
+        instance: [0, 1, 2, 3, 4]
         pg-version: ['9.6']
 
     env:
       # XXX: MATRIX_INSTANCE_TOTAL must be hardcoded to the length of strategy.matrix.instance.
-      MATRIX_INSTANCE_TOTAL: 4
+      MATRIX_INSTANCE_TOTAL: 5
 
     steps:
       - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8  # v3.1.0