builtin:fetchurl: Revert impureEnvVars attribute

This was changed in NixOS#10611, which caused the derivation paths of
anything using builtin:fetchurl to change (i.e. all of
Nixpkgs). However, impureEnvVars doesn't actually do anything for
builtin:fetchurl, so we can just set it to its historical value.

src/libexpr/fetchurl.nix

@@ -28,13 +28,9 @@ derivation ({
   # No need to double the amount of network traffic
   preferLocalBuild = true;
 
+  # This attribute does nothing; it's here to avoid changing evaluation results.
   impureEnvVars = [
-    # We borrow these environment variables from the caller to allow
-    # easy proxy configuration.  This is impure, but a fixed-output
-    # derivation like fetchurl is allowed to do so since its result is
-    # by definition pure.
     "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
-    "HTTP_PROXY" "HTTPS_PROXY" "FTP_PROXY" "ALL_PROXY" "NO_PROXY"
   ];
 
   # To make "nix-prefetch-url" work.