use script_path for webui root in launch_utils

[w-e-w]

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Description

webui modules like launch to be imported correctly in extensions install.py when user run webui from a different cwd then webui root fix

• https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/15697

more sensible implementation of what is trying to achieve

• https://github.com/AUTOMATIC1111/stable-diffusion-webui/pull/15704

script_path is difined relative to modules/paths_internal.p so won't be affected by cwd like the current os.path.abspath('.') https://github.com/AUTOMATIC1111/stable-diffusion-webui/blob/1c0a0c4c26f78c32095ebc7f8af82f5c04fca8c0/modules/paths_internal.py#L16-L17

additional also change the path for CHANGELOG.md

Checklist:

• [x] I have read contributing wiki page
• [x] I have performed a self-review of my own code
• [x] My code follows the style guidelines
• [x] My code passes tests