substitute: Honor "substitute-urls" option passed by "untrusted" clients.

substitute: Honor "substitute-urls" option passed by "untrusted" clients.

* guix/scripts/substitute.scm (or*): New macro.
  (%cache-url): Honor "untrusted-substitute-urls".
* guix/tests.scm (%test-substitute-urls): New variable.
  (open-connection-for-tests): Use it.
* tests/derivations.scm ("derivation-prerequisites-to-build and substitutes",
  "derivation-prerequisites-to-build and substitutes, non-substitutable
  build", "derivation-prerequisites-to-build and substitutes, local build"):
  Pass it to 'set-build-options'.
* tests/guix-daemon.sh: Likewise.
* tests/store.scm ("substitute query, alternating URLs"): New test.
  ("substitute query", "substitute", "substitute + build-things with output
  path", "substitute, corrupt output hash", "substitute --fallback"): Pass
  #:substitute-urls to 'set-build-options'.