Fixes deploy tokens build variables

It was using name, instead of username. Fixes documentation as well

Closes #46454

app/models/ci/build.rb

@@ -661,7 +661,7 @@ module Ci
       Gitlab::Ci::Variables::Collection.new.tap do |variables|
         break variables unless gitlab_deploy_token
 
-        variables.append(key: 'CI_DEPLOY_USER', value: gitlab_deploy_token.name)
+        variables.append(key: 'CI_DEPLOY_USER', value: gitlab_deploy_token.username)
         variables.append(key: 'CI_DEPLOY_PASSWORD', value: gitlab_deploy_token.token, public: false)
       end
     end

changelogs/unreleased/46454-wrong-value-in-ci-deploy-user.yml

+---
+title: Fixes deploy token variables on Ci::Build
+merge_request: 19047
+author:
+type: fixed

doc/user/project/deploy_tokens/index.md

@@ -76,7 +76,7 @@ pull images from your Container Registry.
 > [Introduced][ce-18414] in GitLab 10.8.
 
 There's a special case when it comes to Deploy Tokens, if a user creates one
-named `gitlab-deploy-token`, the name and token of the Deploy Token will be 
+named `gitlab-deploy-token`, the username and token of the Deploy Token will be 
 automatically exposed to the CI/CD jobs as environment variables: `CI_DEPLOY_USER` and 
 `CI_DEPLOY_PASSWORD`, respectively.
 

spec/models/ci/build_spec.rb

@@ -2084,7 +2084,7 @@ describe Ci::Build do
 
       let(:deploy_token_variables) do
         [
-          { key: 'CI_DEPLOY_USER', value: deploy_token.name, public: true },
+          { key: 'CI_DEPLOY_USER', value: deploy_token.username, public: true },
           { key: 'CI_DEPLOY_PASSWORD', value: deploy_token.token, public: false }
         ]
       end