fix: prepare-web build script

[isosphere]

remove quotes from version, but also handle absent directories.

remove directory traversal, use cli options

credit is due to @nebulade for #1129; I did not check for their PR before making this, but I did take their use of a command line arg to strip out the quotations. I was using sed, but this is better.

Thank you so much for your contribution to FluffyChat ❤️❤️❤️

Please make sure that your Pull Request meet the following acceptance criteria:

• The commit message uses the format of Conventional Commits
• The commit message describes what has been changed, why it has been changed and how it has been changed

N/A:

• Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

• Android
• iOS
• Browser (Chromium based)
• Browser (Firefox based)
• Browser (WebKit based)
• Desktop Linux
• Desktop Windows
• Desktop macOS

[isosphere]

re: CI failure:

This request was automatically failed because there were no enabled runners online to process the request for more than 1 days.