Update stage-2-test.yaml with pinned version of cache action

Co-authored-by: Gareth Allan <157592212+gareth-allan@users.noreply.github.com>

Author: Ian-Hodges

.github/workflows/stage-2-test.yaml

@@ -109,7 +109,7 @@ jobs:
           cache-dependency-path: '**/requirements*.txt'
       - name: "Cache generated dependencies"
         id: schema-cache
-        uses: actions/cache@v4
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
         with:
           path: |
             schemas/digital-letters/