Backend: Disable expansion of file type variables when referenced in gitlab-ci variables section

Summary

When using file variables in the "variables" section of a job in .gitlab-ci.yml the contents are being written to the new variable.

Steps to reproduce

The following gitlab-ci.yml

.deployment:
   image: alpine:latest
   script:
      - kubectl config set-cluster ${KUBE_CLUSTER} --server=${KUBE_SERVER} --certificate-authority="${KUBE_CA}"

"Deploy to stage":
   extends: .deployment
   stage: deploy:stage
   when: manual
   variables:
      KUBE_CA: $KUBE_CA_STAGE

"Deploy to prod":
   extends: .deployment
   stage: deploy:prod
   when: manual
   variables:
      KUBE_CA: $KUBE_CA_PROD

The variable KUBE_CA_PROD / KUBE_CA_STAGE are file variables in Gitlab. When using the variable KUBE_CA the contents are no longer a path to a file but the content of the file.

What is the current bug behavior?

Contents of file variables are written to variables

What is the expected correct behavior?

Path of file variable is used as variable

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Ubuntu 18.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.5.3p105
Gem Version:    2.7.9
Bundler Version:1.17.3
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.21.0
Sidekiq Version:5.2.7
Go Version:     unknown
GitLab information
Version:        11.11.2-ee
Revision:       cfdecb7c5de
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     9.6.11
URL:            https://gitlab.XXXXX.com
HTTP Clone URL: https://gitlab.XXXXX.com/some-group/some-project.git
SSH Clone URL:  git@gitlab.XXXXX.com:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     yes
Using Omniauth: yes
Omniauth Providers:
GitLab Shell
Version:        9.1.0
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

not relevant

Proposal

Step	Status
1. GitLab: Stop expanding file variables when sending to Runner	#365859 (closed)
2. Runner: Refactor the creation of the job temporary file path	gitlab-runner#29128
3. Fix file variables in Runner	👈 You are here

Technical

#29407 (comment 935276800)

Note

TODO: Update the docs in !75477 (merged) when this issue is closed.