Backend: Push options are ignored for merge request pipelines
### Summary
The following two features don't seem to work together
* https://docs.gitlab.com/ee/ci/merge_request_pipelines/
* https://docs.gitlab.com/ee/user/project/push_options.html#push-options-for-gitlab-cicd
I would like to use the push options
```bash
git push -o ci.variable="MY_AWESOME_VAR=something"
```
for merge request pipeline but it doesn't work.
### Steps to reproduce
* Create a project
* Use the following `.gitlab-ci.yml`
```yaml
include:
- template: 'Workflows/MergeRequest-Pipelines.gitlab-ci.yml'
image: busybox:latest
show_env:
stage: build
script:
- env | grep AWESOME
```
* Create a merge request
* Push to the merge request using `git push -o ci.variable="MY_AWESOME_VAR=something"`
### Example Project
https://gitlab.com/philipp-rs/pushoptions/-/merge_requests/1
### What is the current *bug* behavior?
The `ci.skip` or `ci.variable` push options have no effect
### What is the expected *correct* behavior?
Push options have an effect as per documentation
issue