describe: Save the original value of (program-arguments).

Fixes <https://bugs.gnu.org/42688>.
Reported by pkill9 <pkill9@runbox.com>.

This ensures that 'guix repl -s SCRIPT' give SCRIPT the right value
of (current-profile), which in turn ensures that (%package-module-path)
is initialized with the right set of channels.

* guix/describe.scm (initial-program-arguments): New variable.
(current-profile): Use it.
* guix/scripts/repl.scm (guix-repl): Call 'current-profile' before
'set-program-arguments'.

guix/describe.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -43,11 +43,17 @@
 ;;;
 ;;; Code:
 
+(define initial-program-arguments
+  ;; Save the initial program arguments.  This allows us to see the "real"
+  ;; 'guix' program, even if 'guix repl -s' calls 'set-program-arguments'
+  ;; later on.
+  (program-arguments))
+
 (define current-profile
   (mlambda ()
     "Return the profile (created by 'guix pull') the calling process lives in,
 or #f if this is not applicable."
-    (match (command-line)
+    (match initial-program-arguments
       ((program . _)
        (and (string-suffix? "/bin/guix" program)
             ;; Note: We want to do _lexical dot-dot resolution_.  Using ".."

guix/scripts/repl.scm

@@ -27,6 +27,7 @@
   #:use-module (srfi srfi-37)
   #:use-module (ice-9 match)
   #:use-module (rnrs bytevectors)
+  #:autoload   (guix describe) (current-profile)
   #:autoload   (system repl repl) (start-repl)
   #:autoload   (system repl server)
                   (make-tcp-server-socket make-unix-domain-server-socket)
@@ -176,6 +177,13 @@ call THUNK."
       ;; Run script
       (save-module-excursion
        (lambda ()
+         ;; Invoke 'current-profile' so that it memoizes the correct value
+         ;; based on (program-arguments), before we call
+         ;; 'set-program-arguments'.  This in turn ensures that
+         ;; (%package-module-path) will contain entries for the channels
+         ;; available in the current profile.
+         (current-profile)
+
          (set-program-arguments script)
          (set-user-module)