test: Add Node 20 to CI matrix

As a first step in the upgrade to Node 20, add it to the CI matrix as a
non-blocking test.

.github/workflows/ci.yml

@@ -9,18 +9,20 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node: [18, 20]
+    continue-on-error: ${{ matrix.node == 20 }}
 
     steps:
     - name: Checkout
       uses: actions/checkout@v4
       with:
         fetch-depth: 0
-    - name: Setup Nodejs Env
-      run: echo "NODE_VER=`cat .nvmrc`" >> $GITHUB_ENV
     - name: Setup Nodejs
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v4
       with:
-        node-version: ${{ env.NODE_VER }}
+        node-version: ${{ matrix.node }}
     - name: Install dependencies
       run: npm ci
     - name: Lint