Specify substitute url in GUI installer

[Zhu Zihao]

Hi, Guix users!

Thanks to the <Alex Chi iskyzh@gmail.com> (from SJTU). We finally set up
a Guix substitute mirror in SJTU LUG.

You can try with command line option
"--substitute-urls=https://mirrors.sjtug.sjtu.edu.cn/guix";

However, it's sad that there's no option for user to specify substitute
url in GUI installer of Guix ISO. It means that people new to Guix still
have to tolerate the slow connection during the installation.

Can we add such option for GUI installer?

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Zihao

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