Always stub find_executable! in specs

What does this merge request do and why?

This always stubs the find_executable! helper to return 'nil', since actually testing if an executable is present depends on the environment and thus might make some tests flaky.

Closes #2199 (closed)

How to set up and validate locally

1. Have a bao binary present in PATH
2. Run be rspec --seed 369
3. See that all tests pass

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.