Rely on GITHUB_ACTIONS environment variable

The Actions platform sets this environment variable automatically. The
documentation says:

> Always set to true when GitHub Actions is running the workflow. You
> can use this variable to differentiate when tests are being run locally
> or by GitHub Actions.

which sounds perfect for our use case.

.github/workflows/ci.yml

@@ -44,7 +44,6 @@ jobs:
       RAILS_VERSION: ${{ matrix.rails }}
       POSTGRES_USER: "postgres"
       POSTGRES_PASSWORD: "postgres"
-      CI: "true"
 
     steps:
       - name: Checkout

bin/setup

@@ -3,7 +3,7 @@
 set -e
 
 # CI-specific setup
-if [ -n "$CI" ]; then
+if [ -n "$GITHUB_ACTIONS" ]; then
   bundle config path vendor/bundle
   bundle config jobs 4
   bundle config retry 3

spec/dummy/config/database.yml

@@ -4,7 +4,7 @@ development: &default
   encoding: unicode
   host: localhost
   pool: 5
-  <% if ENV.fetch("CI", false) %>
+  <% if ENV.fetch("GITHUB_ACTIONS", false) %>
   username: <%= ENV.fetch("POSTGRES_USER") %>
   password: <%= ENV.fetch("POSTGRES_PASSWORD") %>
   <% end %>