Update setup-node GitHub action

The build isn't working because apparently there was a minor security vulnerability in setting the path with the old action (I think).

Here's the failure: https://github.com/austinjavascript/austinjavascript.com/runs/2278717518?check_suite_focus=true

Author: aaronj1335

.github/workflows/eleventy_build.yml

@@ -19,7 +19,7 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Setup Node.js environment
-      uses: actions/setup-node@v1.4.1
+      uses: actions/setup-node@v2
 
     - name: Install packages
       run: npm ci