Mirror in devtools problem (network condition and correctness of rebuild)

[CoelacanthusHex]

We use https://archriscv.felixc.at/ as the default mirror in devtools now. But this tier-0 origin has bad network conditions somewhere in the world, such as in China Mainland. Sometimes it will be into timeout. And often have bed network speed.
Fortunately, Arch Linux provides us with a mirror(https://riscv.mirror.pkgbuild.com/) that is synchronized in time and has good network conditions around the world.
Unfortunately, this can break rebuild because of the tiny sync gap between the mirror and the origin.

Yes. Builders using a not up-to-date mirror may rebuild against an old version of a just-updated dependency.
Originally posted by @felixonmars in #2574 (comment)

Felix Yan also mentioned that one solution is to support overriding the mirrors per builder.