See https://github.com/termux/termux-packages/issues/5620.
Implements automatic mirror selection:
* Pick a random URL of top 4 mirrors of Termux main repository and use it during 30 minutes limit, then rotate to a new one. Distribute traffic accross multiple mirrors, including origin Bintray repository.
* During 30 minutes limit, let `pkg` to check whether mirror is accessible on each run. If mirror is not accessible, use a new one. Mirrors are not guaranteed to be 100% accessible, so let pkg to check which one is working before using `apt`.
* Skip rotating if using `.cn` mirrors - Chinese users will not want to use something else due to great firewall and download speed issues.
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