ci: add pull of session-file-server submodule

.github/workflows/pull-request.yml

@@ -24,6 +24,15 @@ jobs:
       - name: Checkout git repo
         uses: actions/checkout@v2
 
+      - name: Pull git submodules
+        run: git submodule update --init
+
+      - name: Install file server dependency
+        run: |
+          cd session-file-server
+          yarn install;
+          cd -
+
       - name: Install node
         uses: actions/setup-node@v1
         with: