Browser installer lands on // because the profile finish_url has a leading slash

Problem/Motivation

After finishing the browser installer, the site lands on a doubled slash: https://example.com// instead of https://example.com/.

varbase.info.yml declares the redirect with a leading slash:

distribution:
  name: Varbase
  install:
    theme: varbase_installer_theme
    finish_url: '/'

Core's install_goto() prepends the base path, so a value starting with a slash is doubled. drupal_cms_installer declares its own without one (finish_url: 'admin/dashboard/welcome'), which is the convention to match.

This profile value is the one that fires, not the site template's. Core reads only the profile, in core/includes/install.core.inc:

$finish_url = '';
if (isset($install_state['profile_info']['distribution']['install']['finish_url'])) {
  $finish_url = $install_state['profile_info']['distribution']['install']['finish_url'];
}
install_goto($finish_url);

A site template's extra.recipe_installer_kit.finish_url is carried in the installer's query string, but it does not drive this final redirect. So removing the leading slash from the site template recipes (done in Varbase Starter #3614795, Educare #3614796 and Horizon Aid #3614797) is correct for consistency but does not remove the // on its own. This issue is the actual fix.

Steps to reproduce

1. Build a Varbase 11 project and drop the database.
2. Go to /core/install.php, pick a site template, and complete the installer.
3. When the batch finishes the browser lands on https://example.com//.

Proposed resolution

Drop the leading slash, keeping the key present:

    # No leading slash: install_goto() prepends the base path, so '/' yields '//'.
    finish_url: ''

Verified on a browser install: the site then lands on https://example.com/.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
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• ✅ Release notes snippet
• ✅ Release

User interface changes

• After finishing the browser installer the site lands on / rather than //.

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Fixed the browser installer landing on a doubled slash after the install finished.