Fix top calculation for trigger job page sidebar

What does this MR do and why?

The right sidebar's top was hardcoded but this doesn't take into account if the header height changes with/without the existence of the performance bar. This MR fixes that.

No changelog since this feature is under a feature flag.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the ci_retry_downstream_pipeline feature flag. (rails c then Feature.enable(:ci_retry_downstream_pipeline))
2. Add the following content to your .gitlab-ci.yml file:

   stages:
     - prepare
     - deploy
   
   job:
     stage: prepare
     script:
       - echo "running prepare_job"
   
   triggerJob:
     stage: deploy
     trigger:
       include:
         - project: "/path/to/project" # replace with another project
           file: ".gitlab-ci.yml"

3. Visit the pipeline and click on the triggerJob job pill.
4. Site should redirect to the trigger show page.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.