Combine constants into CI_JOB_USER

There is no advantage to two constants. The registry login also uses
find_for_git_client (via JwtController), and the registry login also
uses the job token as the passsword. So just use CI_JOB_USERNAME for
user and job token for password in all cases.

app/models/ci/build.rb

@@ -535,7 +535,7 @@ module Ci
           .append(key: 'CI_JOB_TOKEN', value: token.to_s, public: false, masked: true)
           .append(key: 'CI_BUILD_ID', value: id.to_s)
           .append(key: 'CI_BUILD_TOKEN', value: token.to_s, public: false, masked: true)
-          .append(key: 'CI_REGISTRY_USER', value: ::Gitlab::Auth::CI_REGISTRY_USER)
+          .append(key: 'CI_REGISTRY_USER', value: ::Gitlab::Auth::CI_JOB_USER)
           .append(key: 'CI_REGISTRY_PASSWORD', value: token.to_s, public: false, masked: true)
           .append(key: 'CI_REPOSITORY_URL', value: repo_url.to_s, public: false)
           .concat(deploy_token_variables)

lib/gitlab/auth.rb

@@ -27,7 +27,6 @@ module Gitlab
     DEFAULT_SCOPES = [:api].freeze
 
     CI_JOB_USER = 'gitlab-ci-token'
-    CI_REGISTRY_USER = 'gitlab-ci-token'
 
     class << self
       prepend_if_ee('EE::Gitlab::Auth') # rubocop: disable Cop/InjectEnterpriseEditionModule
@@ -129,7 +128,7 @@ module Gitlab
       # rubocop:enable Gitlab/RailsLogger
 
       def skip_rate_limit?(login:)
-        CI_REGISTRY_USER == login
+        CI_JOB_USER == login
       end
 
       def look_to_limit_user(actor)

lib/gitlab/auth/auth_finders.rb

@@ -82,7 +82,7 @@ module Gitlab
 
         login, password = user_name_and_password(current_request)
         return unless login.present? && password.present?
-        return unless ::Gitlab::Auth::CI_REGISTRY_USER == login
+        return unless ::Gitlab::Auth::CI_JOB_USER == login
 
         job = ::Ci::Build.find_by_token(password)
         raise UnauthorizedError unless job

spec/lib/gitlab/auth/auth_finders_spec.rb

@@ -554,7 +554,7 @@ RSpec.describe Gitlab::Auth::AuthFinders do
     end
 
     context 'with CI username' do
-      let(:username) { ::Gitlab::Auth::CI_REGISTRY_USER }
+      let(:username) { ::Gitlab::Auth::CI_JOB_USER }
       let(:user) { create(:user) }
       let(:build) { create(:ci_build, user: user) }
 
@@ -727,7 +727,7 @@ RSpec.describe Gitlab::Auth::AuthFinders do
 
     context 'when the job token is provided via basic auth' do
       let(:route_authentication_setting) { { job_token_allowed: :basic_auth } }
-      let(:username) { ::Gitlab::Auth::CI_REGISTRY_USER }
+      let(:username) { ::Gitlab::Auth::CI_JOB_USER }
       let(:token) { job.token }
 
       before do

spec/support/helpers/http_basic_auth_helpers.rb

@@ -8,7 +8,7 @@ module HttpBasicAuthHelpers
   end
 
   def job_basic_auth_header(job)
-    basic_auth_header(::Gitlab::Auth::CI_REGISTRY_USER, job.token)
+    basic_auth_header(::Gitlab::Auth::CI_JOB_USER, job.token)
   end
 
   def client_basic_auth_header(client)