tools: use Node.js 16.x for GitHub workflow

find-inactive-collaborators.mjs works fine with Node.js 16.x, but GitHub
Actions currently use 14.x by default.

Author: Trott

.github/workflows/find-inactive-collaborators.yml

@@ -14,4 +14,11 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - run: tools/find-inactive-collaborators.mjs '1 year ago'
+    
+    - name: Install Node.js
+        uses: actions/setup-node@v2
+        with:
+          node-version: 16.x
+
+    - name: Find inactive collaborators
+      run: tools/find-inactive-collaborators.mjs '1 year ago'