[5.4] Always use a new installer instance on discover install

[laoneo]

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 reverts the change from #47637 as it caused issues when installing a package with the web interface. Instead of it moves the installer instantiation out of the loop in the cli console command. Like that no old settings are used from the previous installed extension, when installing multiple at once.

Testing Instructions

• Extract the file plg_system_test1.zip into the folder /plugins/system/test1
• Extract the file plg_system_test2.zip into the folder /plugins/system/test2
• Discover install the extensions on the command line with the commands:
  /cli/joomla.php extension:discover
/cli/joomla.php extension:discover:install

Actual result BEFORE applying this Pull Request

The following info text is displayed:
Postflight executed for test 1

Expected result AFTER applying this Pull Request

The following info text is displayed:
Postflight executed for test 1
Postflight executed for test 2

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[joomdonation]

I have tested this item ✅ successfully on 04fbff3

Both issues are sorted

• Installation script is executed properly when installing extension via web interface
• Installation script for the two plugins are executed properly with discover install via CLI

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[richard67]

@joomdonation Could you test again with the latest changes? Thanks in advance.

[roland-d]

I have tested this item ✅ successfully on 5185894

So I have tested the changes and they work as expected. Before applying the patch my script.php was not found and thus not executed. After applying the patch my script.php is found and properly executed.

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[joomdonation]

I have tested this item ✅ successfully on 5185894

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[joomdonation]

RTC

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[richard67]

Thanks @laoneo for this PR, @roland-d and @joomdonation for testing, and @QuyTon for review.