chore(ci): Use GH's setup-node Action

Since actions/setup-node#338, GH's setup-node Action understands `.nvmrc` files.

This makes our `guardian/actions-setup-node` fork somewhat redundant now.

Move to GH's Action so we can deprecate our fork.

.github/workflows/cd.yaml

@@ -9,9 +9,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Setup node
-        uses: guardian/actions-setup-node@v2.4.1
+      - uses: actions/setup-node@v2
         with:
+          node-version-file: '.nvmrc'
           cache: npm
       - name: CD
         env:

.github/workflows/ci.yaml

@@ -26,9 +26,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Setup node
-        uses: guardian/actions-setup-node@v2.4.1
+      - uses: actions/setup-node@v2
         with:
+          node-version-file: '.nvmrc'
           cache: npm
       - run: ./script/ci
   approve-and-merge: