refactor(ci): move node to env var, move env var up

So it can be used by all jobs.

.github/workflows/ci.yml

@@ -8,14 +8,15 @@ on:
       - "trying"
   pull_request:
 
-jobs:
-  build_and_test:
 env:
   BUF_VERSION: "0.56.0"
   NIM_WAKU_VERSION: "v0.9"
+  NODE_JS: "16"
+
+jobs:
+  build_and_test:
     strategy:
       matrix:
-        node: [16]
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps:
@@ -33,7 +34,7 @@ jobs:
       - name: Install NodeJS
         uses: actions/setup-node@v3
         with:
-          node-version: ${{ matrix.node }}
+          node-version: ${{ env.NODE_JS }}
 
       - name: Ensure wakunode2 is ready
         shell: bash