GitHub workflows: Install Node.js v12

I'm not sure how these tests ever worked. I guess some version of
Node.js and npm come pre-installed on the ubuntu-latest images?

I would have prefered to use Node.js v10 because that is our current
minimum supported version, but we have a surprising number of tests
that don't work on Node.js v10 (mostly due to `assert.match()`, which
was added in Node.js v12).

.github/workflows/load-test.yml

@@ -17,6 +17,10 @@ jobs:
     - name: Checkout repository
       uses: actions/checkout@v2
 
+    - uses: actions/setup-node@v2
+      with:
+        node-version: 12
+
     - name: Install all dependencies and symlink for ep_etherpad-lite
       run: src/bin/installDeps.sh
 
@@ -39,6 +43,10 @@ jobs:
     - name: Checkout repository
       uses: actions/checkout@v2
 
+    - uses: actions/setup-node@v2
+      with:
+        node-version: 12
+
     - name: Install etherpad-load-test
       run: sudo npm install -g etherpad-load-test