fetchurlBoot: Use Nix's builtin fetchurl function

This removes the need for curl in bootstrapTools, and enables https
for bootstrap tarballs.

pkgs/build-support/fetchurl/boot.nix

@@ -0,0 +1,20 @@
+let mirrors = import ./mirrors.nix; in
+
+{ system }:
+
+{ url ? builtins.head urls
+, urls ? []
+, sha256
+}:
+
+import <nix/fetchurl.nix> {
+  inherit system sha256;
+
+  url =
+    # Handle mirror:// URIs. Since <nix/fetchurl.nix> currently
+    # supports only one URI, use the first listed mirror.
+    let m = builtins.match "mirror://([a-z]+)/(.*)" url; in
+    if m == null then url
+    else builtins.head (mirrors.${builtins.elemAt m 0}) + (builtins.elemAt m 1);
+
+}