Minor improvements to CI_API_V4_URL variable exposure

changelogs/unreleased/feature-gb-expose-ci-api-url-variable.yml

 ---
-title: Expose CI/CD predefined variable
+title: Expose CI/CD predefined variable `CI_API_V4_URL`
 merge_request: 23936
 author:
 type: added

lib/api/helpers/version.rb

@@ -18,7 +18,7 @@ module API
       end
 
       def root_url
-        expose_url(root_path)
+       @root_url ||= expose_url(root_path)
       end
 
       def to_s

spec/models/project_spec.rb

@@ -3504,7 +3504,7 @@ describe Project do
       expect(project.api_variables.first[:value]).to include '/api/v4'
     end
 
-    it 'contains an URL variable for every supported API version' do
+    it 'contains a URL variable for every supported API version' do
       supported_versions = API::API.versions.select do |version|
         API::API.routes.select { |route| route.version == version }.many?
       end