GitHub Actions: Use latest ruby/setup-ruby@v1 action

THe old pinned version doesn't work anymore due to changes in the action
and the environment. It used set-env which is now deprecated: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

.github/workflows/test-build.yml

@@ -19,10 +19,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby
-    # To automatically get bug fixes and new Ruby versions for ruby/setup-ruby,
-    # change this to (see https://github.com/ruby/setup-ruby#versioning):
-    # uses: ruby/setup-ruby@v1
-      uses: ruby/setup-ruby@ec106b438a1ff6ff109590de34ddc62c540232e0
+      uses: ruby/setup-ruby@v1
       with:
         ruby-version: 2.7
     - uses: actions/cache@v2