Extract literal string into constant

This string was used in two places but they are definitely related.
repo_url is building the basic auth string that will be acceped by
build_access_token_check

app/models/ci/build.rb

@@ -596,7 +596,7 @@ module Ci
     def repo_url
       return unless token
 
-      auth = "gitlab-ci-token:#{token}@"
+      auth = "#{::Gitlab::Auth::CI_JOB_USER}:#{token}@"
       project.http_url_to_repo.sub(%r{^https?://}) do |prefix|
         prefix + auth
       end

lib/gitlab/auth.rb

@@ -26,6 +26,8 @@ module Gitlab
     # Default scopes for OAuth applications that don't define their own
     DEFAULT_SCOPES = [:api].freeze
 
+    CI_JOB_USER = 'gitlab-ci-token'
+
     class << self
       prepend_if_ee('EE::Gitlab::Auth') # rubocop: disable Cop/InjectEnterpriseEditionModule
 
@@ -254,7 +256,7 @@ module Gitlab
       end
 
       def build_access_token_check(login, password)
-        return unless login == 'gitlab-ci-token'
+        return unless login == CI_JOB_USER
         return unless password
 
         build = find_build_by_token(password)

spec/lib/gitlab/auth_spec.rb

@@ -149,7 +149,9 @@ RSpec.describe Gitlab::Auth, :use_clean_rails_memory_store_caching do
     end
 
     context 'build token' do
-      subject { gl_auth.find_for_git_client('gitlab-ci-token', build.token, project: project, ip: 'ip') }
+      subject { gl_auth.find_for_git_client(username, build.token, project: project, ip: 'ip') }
+
+      let(:username) { 'gitlab-ci-token' }
 
       context 'for running build' do
         let!(:build) { create(:ci_build, :running) }
@@ -170,6 +172,14 @@ RSpec.describe Gitlab::Auth, :use_clean_rails_memory_store_caching do
 
           expect(subject).to eq(Gitlab::Auth::Result.new(nil, nil, nil, nil))
         end
+
+        context 'username is not gitlab-ci-token' do
+          let(:username) { 'another_username' }
+
+          it 'fails to authenticate' do
+            expect(subject).to eq(Gitlab::Auth::Result.new(nil, nil, nil, nil))
+          end
+        end
       end
 
       (Ci::HasStatus::AVAILABLE_STATUSES - ['running']).each do |build_status|