Fix Copilot app download link for Linux ARM64 (#64582)

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: saritai

data/reusables/copilot/github-app-download-button.md

@@ -1 +1 @@
-<a href="https://github.com/features/ai/github-app?utm_source=app-download-docs&utm_medium=docs&utm_campaign=universe-2026" target="_blank" class="btn btn-primary mt-3 mr-3 no-underline js-os-download" data-os-download-mac="https://gh.io/copilot-app-mac" data-os-download-mac-intel="https://gh.io/copilot-app-mac-intel" data-os-download-windows="https://gh.io/copilot-app-win64" data-os-download-windows-arm="https://gh.io/copilot-app-win-arm" data-os-download-linux="https://gh.io/copilot-app-linux">{% octicon "desktop-download" aria-hidden="true" %} <span class="js-os-download-label">Download the {% data variables.copilot.github_copilot_app %}</span></a>
+<a href="https://github.com/features/ai/github-app?utm_source=app-download-docs&utm_medium=docs&utm_campaign=universe-2026" target="_blank" class="btn btn-primary mt-3 mr-3 no-underline js-os-download" data-os-download-mac="https://gh.io/copilot-app-mac" data-os-download-mac-intel="https://gh.io/copilot-app-mac-intel" data-os-download-windows="https://gh.io/copilot-app-win64" data-os-download-windows-arm="https://gh.io/copilot-app-win-arm" data-os-download-linux="https://gh.io/copilot-app-linux" data-os-download-linux-arm="https://github.com/github/app/releases/latest/download/GitHub-Copilot-linux-arm64.AppImage">{% octicon "desktop-download" aria-hidden="true" %} <span class="js-os-download-label">Download the {% data variables.copilot.github_copilot_app %}</span></a>