CI: Introduce RAILS_MASTER_KEY placeholder

Improves existing CI templates by adding an [environment variable][ev]
to store the `RAILS_MASTER_KEY` which should be stored as a [secret][].

This is necessary when [`config.require_master_key`][crmk] is enabled,
or when using [custom credentials][cc].

Since those features are not enabled by default, we comment this value
out, which is consistent with how we treat Redis.

[ev]: https://docs.github.com/en/actions/learn-github-actions/variables
[secret]: https://docs.github.com/en/actions/security-guides/using-secrets-in-github-actions
[crmk]: https://guides.rubyonrails.org/configuring.html#config-require-master-key
[cc]: https://guides.rubyonrails.org/security.html#custom-credentials

Author: stevepolitodesign

railties/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Introduce `RAILS_MASTER_KEY` placeholder in generated ci.yml files
+
+    *Steve Polito*
+
 *   Colorize the Rails console prompt even on non standard environments.
 
     *Lorenzo Zabot*

railties/lib/rails/generators/rails/app/templates/github/ci.yml.tt

@@ -127,6 +127,7 @@ jobs:
           <%- elsif options[:database] == "postgresql" -%>
           DATABASE_URL: postgres://postgres:postgres@localhost:5432
           <%- end -%>
+          # RAILS_MASTER_KEY: ${{ secrets.RAILS_MASTER_KEY }}
           # REDIS_URL: redis://localhost:6379/0
         <%- if options[:api] || options[:skip_system_test] -%>
         run: bin/rails db:test:prepare test

railties/lib/rails/generators/rails/plugin/templates/github/ci.yml.tt

@@ -91,6 +91,7 @@ jobs:
           <%- elsif options[:database] == "postgresql" -%>
           DATABASE_URL: postgres://postgres:postgres@localhost:5432
           <%- end -%>
+          # RAILS_MASTER_KEY: ${{ secrets.RAILS_MASTER_KEY }}
           # REDIS_URL: redis://localhost:6379/0
         run: <%= test_command %>