fix: #3614810 Remove the leading slash from the profile finish_url

Issue: https://www.drupal.org/project/varbase/issues/3614810

After the browser installer finishes, the site lands on https://example.com//.

varbase.info.yml declares distribution.install.finish_url: '/', and core's install_goto() prepends the base path, so the slash is doubled.

This profile value is the one that fires. 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 rides in the installer's query string but does not drive this redirect, so the matching recipe fixes (Varbase Starter #3614795, Educare #3614796, Horizon Aid #3614797) are right for consistency but do not remove the //. This is the fix that does.

The key stays present with an empty value, matching drupal_cms_installer's convention of declaring it without a leading slash. Verified on a browser install: the site lands on https://example.com/.

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