Preserve relative order of overridden variables

What does this MR do?

This MR fixes an issue found during the last deployment of the :variable_inside_variable FF (it is currently disabled waiting for this MR). The issue has to do with the new topological sorting that happens on variables when the feature flag is enabled. It can be reproduced when there are multiple instances of a variable with the same name due to it being overridden in parent subgroups. Here is a sample of the ordering when the FF is disabled:

[
  "TEST=$AWS_ACCOUNT_NONPROD",
  "GITLAB_USER_ID=1",
  "GITLAB_USER_EMAIL=admin@example.com",
  "GITLAB_USER_LOGIN=root",
  "GITLAB_USER_NAME=Administrator",
  "AWS_ACCOUNT_NONPROD=gitlab-org",
  "AWS_ACCOUNT_NONPROD=vars-in-vars-subgroup"
]

Once we enable the FF, we see AWS_ACCOUNT_NONPROD correctly being moved before the variable that references it (TEST), but since we keep the definitions of AWS_ACCOUNT_NONPROD from multiple group levels, the definition from parent groups end up overwriting the first value and the top-level group value wins:

[
  "AWS_ACCOUNT_NONPROD=vars-in-vars-subgroup",
  "TEST=vars-in-vars-subgroup",
  "GITLAB_USER_ID=1",
  "GITLAB_USER_EMAIL=admin@example.com",
  "GITLAB_USER_LOGIN=root",
  "GITLAB_USER_NAME=Administrator",
  "AWS_ACCOUNT_NONPROD=gitlab-org"
]

The topological sorting algorithm needs to be improved so that it moves all instances (preserving order) of a referenced value before the reference variable, not just the first one:

[
  "AWS_ACCOUNT_NONPROD=gitlab-org",
  "AWS_ACCOUNT_NONPROD=vars-in-vars-subgroup",
  "TEST=vars-in-vars-subgroup",
  "GITLAB_USER_ID=1",
  "GITLAB_USER_EMAIL=admin@example.com",
  "GITLAB_USER_LOGIN=root",
  "GITLAB_USER_NAME=Administrator"
]

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. In a GDK instance, create the following project structure:

   • toplevel-playground-group
     • playground-subgroup
       • project.

2. In toplevel-playground-group, define AWS_ACCOUNT_NONPROD as top-level value in the group CI Variables page

3. In toplevel-playground-group / playground-subgroup, define AWS_ACCOUNT_NONPROD as subgroup value in the group CI Variables page

4. In project, create a .gitlab-ci.yml file with the following content:

   stages:
     - test
   
   variables:
     AWS_ACCOUNT: "${AWS_ACCOUNT_NONPROD}"
   
   test-job:
     stage: test
     script:
       - |
         echo "AWS_ACCOUNT_NONPROD: $AWS_ACCOUNT_NONPROD"
         echo "AWS_ACCOUNT: $AWS_ACCOUNT"

5. Enable the variable inside variables logic globally in rails console:

   Feature.enable(:variable_inside_variable)

6. Run a pipeline

7. Both AWS_ACCOUNT and AWS_ACCOUNT_NONPROD should appear as subgroup value, which is the same expected result as when the FF is disabled.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Part of gitlab-runner#1809 (closed)