Add asdf-install targets for gitlab-zoekt-indexer and zoekt

What does this merge request do and why?

⚠ This MR needs !3490 (merged) merged first.

Similar to !3454 (merged), this MR introduces gitlab-zoekt-indexer-asdf-install and zoekt-indexer-asdf-install targets. These targets run asdf install in the /gitlab-zoekt-indexer and /zoekt directories, rather than across the entire GDK project. With this change, installing dependencies in /gitlab-zoekt-indexer and /zoekt is not necessary if Zoekt is not enabled.

Related to #1635 (closed)

How to set up and validate locally

1. git checkout 1635-asdf-install-in-gitlab-zoekt-indexer.
2. gdk config set zoekt.enabled true.
3. gdk reconfigure.

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 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.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.