nixpkgs calls <nix/fetchurl.nix> during nixpkgs bootstrap.
This produces a fake derivation with system = builtin
and builder = builtin:fetchurl, and needs to download files from the
internet.
At the end of the Derivation construction, if we have such a derivation,
also synthesize a `Fetch` struct, which we add to the known fetch paths.
This will then cause these fetches to be picked up like all other
fetches in TvixStoreIO.
Change-Id: I72cbca4f85da106b25eda97693a6a6e59911cd57
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10975
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI
