This allows on-the-fly rewriting of URLs before they are passed from
fetchurl (or fetchurlBoot) to curl.
The intended use is to allow inserting company-internal mirrors, or
working around company firewalls and similar network restrictions,
without having to extensively patch across all of nixpkgs. Instead,
users can pass a function in their nixpkgs that performs the necessary
URL rewrites.
Co-authored-by: Alexander Bantyev <balsoft@balsoft.ru>
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