improve error handling flags to curl

current invocation is extremely fragile if tcp sockets are spuriously shutdown (this can happen when changing networks, or just due to a bad ISP)

--retry-all-errors is the only way to make curl robust against this type of failure.

pkgs/build-support/fetchurl/builder.sh

@@ -14,6 +14,8 @@ curl=(
     --location
     --max-redirs 20
     --retry 3
+    --retry-all-errors
+    --continue-at -
     --disable-epsv
     --cookie-jar cookies
     --user-agent "curl/$curlVersion Nixpkgs/$nixpkgsVersion"