hydra-eval-guile-jobs: Honor $NIX_STORE_DIR.

* src/script/hydra-eval-guile-jobs.in (strip-store-path): Honor
  $NIX_STORE_DIR.
hydra-eval-guile-jobs: Register derivations as GC roots.

* src/script/hydra-eval-guile-jobs.in (register-gc-root): New
  procedure.
  (job-evaluations->sxml): Add #:gc-roots-dir parameter.  Call
  'register-gc-root'.
  (job-evaluations->xml): Add #:gc-roots-dir parameter; pass it to
  'job-evaluations->sxml'.
  (eval-guile-jobs): Warn when --gc-roots-dir isn't passed.  Pass
  GC-ROOTS-DIR to 'job-evaluations->xml'.
hydra-eval-guile-jobs: Add "This file is part of Hydra".