Add pnpm to runners (#1683)

Found another place where pnpm needs to be added.

.github/workflows/elixir.yml

@@ -317,6 +317,12 @@ jobs:
         with:
           otp-version: "25"
           elixir-version: "1.14"
+      - uses: actions/setup-node@v3
+        with:
+          node-version: 18
+      - uses: pnpm/action-setup@v2
+        with:
+          version: 8
       - uses: actions/checkout@v3
       - uses: actions/cache@v3
         name: Elixir Deps Cache