gh variable list --json numSelectedRepos provides different results depending on TTY or not #9255

Describe the bug

I think this is the same bug as #8679

The TL;DR is that gh variable list does not correctly handle --json numSelectedRepos when in a non-interactive session. This is because we only fetch the selected repos if we're hooked up to TTY which was correct when the command was introduced but not when --json became supported

Steps to reproduce the behavior

1. Create an organization secret
2. Set it's visibility to "Selected" and choose a repo
3. Run gh variable list -o <org> --json numSelectedRepos and see the value = 1
4. Run gh variable list -o <org> --json numSelectedRepos | cat and see the value = 1