Run gdk install using mise exec or asdf exec

What does this merge request do and why?

This MR fixes the gdk: command not found error in the one-line installation. After bootstrap, mise or asdf is set up, but the shell does not refresh, so the gdk command is not available when gdk install runs. Normally, users restart their shell to apply the changes, but since the one-line installation runs everything in one step, stopping for a manual refresh is not ideal. To fix this, mise exec or asdf exec runs gdk install when enabled, so the command works without the user restarting the shell.

Closes #2483 (closed)

How to set up and validate locally

Run the one-line installation command below and make sure it succeeds:

curl "https://gitlab.com/gitlab-org/gitlab-development-kit/-/raw/2483-fix-gdk-command-missing-one-line-install/support/install" | bash -s - gdk "2483-fix-gdk-command-missing-one-line-install" "mise" "."

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.