[E2E] Modifying certs to use from fixtures for workspace_actions_spec

What does this MR do and why?

This change unblocks the spec to be running in staging environment, until !131738 (merged) and gitlab-qa!1233 (merged) is fixed.

These values were initially defined as file variables, but facing issue in reading file variables inside the gitlab-qa docker containers, so making them as fixtures for now.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/424857

How to set up and validate locally

The full list of variable and their values to run this spec can be found in 1password "Run workspaces tests against staging"

bundle exec bin/qa Test::Instance::All https://staging.gitlab.com -- -- qa/specs/features/ee/browser_ui/3_create/remote_development/workspace_actions_spec.rb

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.

Related to #424857