Shadowsocks FOSS: fix v5.1.2 build

• The app complies with the inclusion criteria
• All related fdroiddata have been referenced in this merge request
• The upstream app source code repo contains the app metadata (summary/description/images/changelog/etc) in a Fastlane or Triple-T folder structure
• Builds with fdroid build
• Releases are tagged

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I will tag the last commit in master branch once this MR was merged. v5.1.2 release is tagged now.

Switched from shadowsocks-libev to shadowsocks-rust so it needs rustup now.

I've tested the build and checkupdates, both works correctly.

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Found some problems, do not merge until I remove [WIP] from the title. Should be solved now, I've test the build inside docker.