Stop reporting an unset BUNDLE_PATH as a failed command

What does this merge request do and why?

gdk doctor runs bundle config get PATH to make sure BUNDLE_PATH is not set. Newer Bundler versions exit 1 when the value is unset, which is the healthy answer here, so Shellout#execute printed "'bundle config get PATH' failed." and retried before the diagnostic went on to report the GDK as healthy.

Run the command without error output or retries, since the exit code carries no information the stdout check does not already give us, and cover the non-zero exit in the spec.

Resolves #3346 (closed)

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, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.

Closes #3346 (closed)