package: Honor '--dry-run' when target profile is already in store.

Fixes <https://issues.guix.gnu.org/53267>.
Reported by Tirifto <tirifto@posteo.cz>.

Regression introduced in 65ffb9388c.

In the (unlikely) case where the profile we're targeting with "guix
upgrade -n" or similar is already built, a new profile generation would
be created and linked to despite the use of '-n'.  This is because
65ffb9388c assumed that dry-run behavior
would be handled solely by the build handler, which is not the case when
there's nothing to build.

* guix/scripts/package.scm (build-and-use-profile): Reintroduce #:dry-run?
and honor it.
(process-actions): Pass #:dry-run? to 'build-and-use-profile'.
* tests/guix-package-net.sh: Add test.

guix/scripts/package.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2013, 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2014, 2016 Alex Kost <alezost@gmail.com>
@@ -139,6 +139,7 @@ denote ranges as interpreted by 'matching-generations'."
 
 (define* (build-and-use-profile store profile manifest
                                 #:key
+                                dry-run?
                                 (hooks %default-profile-hooks)
                                 allow-collisions?
                                 bootstrap?)
@@ -154,6 +155,7 @@ hooks\" run when building the profile."
          (prof     (derivation->output-path prof-drv)))
 
     (cond
+     (dry-run? #t)
      ((and (file-exists? profile)
            (and=> (readlink* profile) (cut string=? prof <>)))
       (format (current-error-port) (G_ "nothing to be done~%")))
@@ -1069,6 +1071,7 @@ processed, #f otherwise."
                                    trans
                                    #:dry-run? dry-run?)
         (build-and-use-profile store profile new
+                               #:dry-run? dry-run?
                                #:allow-collisions? allow-collisions?
                                #:bootstrap? bootstrap?)))))
 

tests/guix-package-net.sh

@@ -196,6 +196,16 @@ EOF
 guix package --bootstrap -p "$profile" -i gcc-bootstrap
 installed="`guix package -p "$profile" -I | cut -f1`"
 
+# Dry-run upgrade.  Make sure no new generation is created when things are
+# already in store and '-n' is used: <https://issues.guix.gnu.org/53267>.
+V_MINOR=0
+export V_MINOR
+profile_before="$(readlink "$profile")"
+guix package -p "$profile" --bootstrap -L "$module_dir" -u # build the profile
+guix package -p "$profile" --roll-back
+guix package -p "$profile" --bootstrap -L "$module_dir" -u . -n # check '-n'
+test "$(readlink "$profile")" = "$profile_before"
+
 for i in 1 2
 do
     V_MINOR="$i"