Support setting environment variables in the inferior

When processing jobs, this is mostly to allow setting GUIX_DOWNLOAD_METHODS.

scripts/guix-data-service-process-job.in

@@ -51,7 +51,12 @@
                   (alist-cons 'parallelism
                               (string->number arg)
                               (alist-delete 'parallelism
-                                            result))))))
+                                            result))))
+        (option '("inferior-set-environment-variable") #t #f
+                (lambda (opt name arg result)
+                  (alist-cons 'inferior-environment-variable
+                              (string-split arg #\=)
+                              result)))))
 
 (define %default-options
   '((parallelism . 1)))
@@ -79,6 +84,13 @@
           (process-load-new-guix-revision-job
            job
            #:skip-system-tests? (assq-ref opts 'skip-system-tests)
+           #:extra-inferior-environment-variables
+           (filter-map
+            (match-lambda
+              (('inferior-environment-variable key val)
+               (cons key val))
+              (_ #f))
+            opts)
            #:parallelism (assq-ref opts 'parallelism)))
         #:hz 0
         #:parallelism 1)))))