fix: bin/sandbox command

Use `bundle exec` to invoke the rails installer,
otherwise it will fail with

```
/Users/<user>/.rubies/ruby-3.3.5/lib/ruby/3.3.0/rubygems.rb:259:in `find_spec_for_exe': can't find gem rails (= 7.2.1.1) with executable rails (Gem::GemNotFoundException)
	from /Users/<user>/.rubies/ruby-3.3.5/lib/ruby/3.3.0/rubygems.rb:278:in `activate_bin_path'
	from /Users/<user>/.gem/ruby/3.3.5/bin/rails:25:in `<main>'
```

although it is installed.

bin/sandbox

@@ -42,7 +42,7 @@ rm -rf ./sandbox
 
 echo "~~~> Creating a pristine Rails app"
 rails_version=`bundle exec ruby -e'require "rails"; puts Rails.version'`
-rails _${rails_version}_ new sandbox \
+bundle exec rails _${rails_version}_ new sandbox \
   --database="$RAILSDB" \
   --skip-git \
   --skip-keeps \