Update command for setting environment variable

What does this Merge Request do and why?

This will update the command for setting the GITLAB_SIMULATE_SAAS environment variable for testing features only available on SaaS (GitLab.com), so that it works for a running workspace in Gitpod.

Cross-posting from relevant discussion in gitlab!86684 (comment 970841083):

To clarify, setting the environment variable for SaaS simulation needs to happen with gp env instead of export to be able to set this within the running workspace, see relevant docs.

Alternatively, you can set up a user-level environment variable in Gitpod user settings[2] or inside the recently added project as project-level environment variable in gitpod.io/t/gitlab-org/gitlab/variables so that every team member with access to the project get this variable by default if needed.

See also relevant discussion (internal) and channel bookmarks in #gitpod-gdk on how to join the gitlab-org team on Gitpod.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.