GitHub Action: Avoid issues with set-env used by older versions of se…

…tup-node

https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
contains:
`Action authors who are using the toolkit should update the @actions/core package to v1.2.6 or greater to get the updated addPath and exportVariable functions.`
and
`Action and workflow authors who are setting environment variables via STDOUT should update any usage of the set-env and add-path workflow commands to use the new environment files.`

Which setup-node appear to have done in actions/setup-node#200
The commits for which have been tagged v2.1.2.

.github/workflows/ruby.yml

@@ -34,7 +34,7 @@ jobs:
       run: bundle install
 
     - name: Setup Node
-      uses: actions/setup-node@v1
+      uses: actions/setup-node@v2.1.2
       with:
         node-version: 12.16.3