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.
authored by Johannes Maier and committed by GitHub 39e6ebdf 73258f34