dependencies: Add checker for nginx

What does this merge request do and why?

When nginx is enabled the executable should be found.

Because some people might have set an absolute path for nginx.bin, this MR also includes a patch that allows for that.

How to set up and validate locally

Run rake preflight-checks in a few different scenarios:

1. gdk config set nginx.enabled false && gdk config set nginx.bin non-existing => preflight checks should PASS.
2. gdk config set nginx.enabled true && gdk config set nginx.bin non-existing => preflight checks should FAIL.
3. gdk config set nginx.enabled true && gdk config set nginx.bin true => preflight checks should 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.