Revert "Only use whatever Node is on my runner"

This reverts commit 375b047.

I'm going to try this (removing cache: 'npm'):

actions/setup-node#706 (comment)

It looks like setup-node isn't supposed to need a package.json present
at the project root.

.github/workflows/node.js.yml

@@ -13,8 +13,19 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
+    strategy:
+      matrix:
+        node-version: [18.x, 20.x]
+        # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
+
     steps:
       - uses: actions/checkout@v4
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v2
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: "npm"
+          working-directory: GitHubOIDCRoleStack
       - name: Install
         working-directory: GitHubOIDCProvider
         run: npm install