fix: prevent install failure when CLI has authenticated with GHE

[epicbugs]

Running shortened installed commands gh extension install github/gh-stack and gh skill install github/gh-stack will throw errors like could not resolve version: could not determine default branch: HTTP 404: Not Found (https://api.<company>.ghe.com/repos/github/gh-stack) when GH CLI is already authenticated with Github Enterprise account.

Why:

Following current installation instruction will lead to errors when GH CLI is already authenticated with Github Enterprise account.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Installation commands for gh extension and gh skill.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
pull-requests/get-started/stacked-prs-quickstart.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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