Set RUBY_VERSION env var in Rails Main workflow

The Gemfile conditionally reads .ruby-version only when RUBY_VERSION
env var is not set.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ndbroadbent

.github/workflows/rails-main.yml

@@ -9,6 +9,8 @@ jobs:
   rails-main:
     name: Rails main integration tests
     runs-on: ubuntu-22.04
+    env:
+      RUBY_VERSION: '4.0.1'
     steps:
       - uses: actions/checkout@v4