Pull Request resolves #47948.

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

This PR fixes a regression where installer scripts defined in extension manifests are not executed during component and package installations.

The installer attempted to load the manifest script from extension_root before the extension files had been copied there. The script is now loaded from the source path, where it exists during the installation process.

Testing Instructions

1. Install a component or package containing a manifest scriptfile.
2. Verify that the installer script methods (preflight, install, update, postflight) are executed correctly.
3. Repeat the installation using this PR.
4. Confirm that migrations and other installer logic defined in the script file run successfully.

Actual result BEFORE applying this Pull Request

The installer script was not executed because Joomla attempted to load it from extension_root before the extension files had been copied there.

This resulted in install steps, migrations, and other custom installation logic not being executed.

Expected result AFTER applying this Pull Request

The installer script is loaded successfully from the temporary source directory during installation, allowing install/update scripts and migrations to execute as expected.

Link to documentations

• No documentation changes for guide.joomla.org needed
• No documentation changes for manual.joomla.org needed