buildDhallUrl: Respect proxy environment variables

`buildDhallUrl` is a fixed output derivation, which means
that we can (and should) inherit impure proxy-related
environment variables for fetching URLs.

The motivation for this change is:

https://discourse.dhall-lang.org/t/cant-build-a-nixified-dhall-package-in-a-sandbox-depending-on-the-environment/

… where a `buildDhallUrl` derivation was failing in a restricted
networking environment due to not inheriting proxy-related
settings.

pkgs/development/interpreters/dhall/build-dhall-url.nix

@@ -76,7 +76,8 @@ let
    sourceFile = "source.dhall";
 
 in
-  runCommand name { } (''
+  runCommand name { impureEnvVars = lib.fetchers.proxyImpureEnvVars; }
+ (''
     set -eu
 
     mkdir -p ${cacheDhall} $out/${cacheDhall}