hydra-eval-guile-jobs: Adjust to <license> objects.

* src/script/hydra-eval-guile-jobs.in (job-evaluations->sxml): Output
  the license name, not the external representation of <license>
  objects.
hydra-eval-guile-jobs: Adjust to multiple-output format.

This is a followup to commit
10882a1ffdbc9c6050c0528cf95be98fb5541d50 ("Add multiple output
support").

* src/script/hydra-eval-guile-jobs.in (job-evaluations->sxml): Return
  several `output' tags in the body, and remove the `outPath' attribute
  of `job'.
hydra-eval-guile-jobs: Allow use of the installed Guix.

* src/script/hydra-eval-guile-jobs.in: Leave GUILE_LOAD_PATH and
  GUILE_LOAD_COMPILED_PATH unchanged.