Support setting environment variables in the inferior

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

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

@@ -24,6 +24,7 @@
 
 (use-modules (srfi srfi-1)
              (srfi srfi-37)
+             (ice-9 match)
              (guix-data-service database)
              (guix-data-service jobs))
 
@@ -49,6 +50,11 @@
                 (lambda (opt name arg result)
                   (alist-cons 'per-job-parallelism
                               (string->number arg)
+                              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
@@ -95,6 +101,13 @@
                     (* 2 (assq-ref opts 'max-processes)))
                 #: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)
                 #:per-job-parallelism
                 (assq-ref opts 'per-job-parallelism)))
            (lambda _