buildDhallUrl: fix impure proxy variable passing (#178544)

PR #177891 tried fixing a problem with `buildDhallUrl` in environments
where proxy variables are necessary for internet access to work. The
`impureEnvVars` should be set in `downloadEncodedFile` instead of the
final `runCommand`, as the former is an FOD, the latter isn't.

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

···
         outputHash = hash;
         name = baseNameOf url;
         nativeBuildInputs = [ cacert ];
+        impureEnvVars = lib.fetchers.proxyImpureEnvVars;
       }
       ''
         echo "${url} ${dhallHash}" > in-dhall-file
···
    sourceFile = "source.dhall";
 
 in
-  runCommand name { impureEnvVars = lib.fetchers.proxyImpureEnvVars; }
+  runCommand name { }
  (''
     set -eu