pack: Make sure tests can run without a world rebuild.

Commit 68380db4c4 moved from
'gexp->derivation', which as a side effect, would lead tests to require
a "world rebuild"--specifically, they'd have to build (default-guile).

This was mitigated by 68775338a5, but that
change introduced another regression.

* guix/scripts/pack.scm (populate-profile-root): Define 'bootstrap?'.
Pass #:guile to 'computed-file', with a value depending on 'bootstrap?'.
* tests/pack.scm ("self-contained-tarball + localstatedir")
("docker-image + localstatedir", "squashfs-image + localstatedir")
("deb archive with symlinks and control files")
("rpm archive can be installed/uninstalled"): Use a <profile> record
instead of a derivation.

guix/scripts/pack.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015, 2017-2022 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2017-2023 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017, 2018 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2018 Konrad Hinsen <konrad.hinsen@fastmail.net>
 ;;; Copyright © 2018 Chris Marusich <cmmarusich@gmail.com>
@@ -220,6 +220,11 @@ items, which relies on hard links."
          (file-append (store-database (list profile))
                       "/db/db.sqlite")))
 
+  (define bootstrap?
+    ;; Whether a '--bootstrap' environment is needed, for testing purposes.
+    ;; XXX: Infer that from available info.
+    (and (not database) (not (profile-locales? profile))))
+
   (define (import-module? module)
     ;; Since we don't use deduplication support in 'populate-store', don't
     ;; import (guix store deduplication) and its dependencies, which includes
@@ -287,6 +292,7 @@ items, which relies on hard links."
           (for-each (cut evaluate-populate-directive <> #$output)
                     directives)))
     #:local-build? #f
+    #:guile (if bootstrap? %bootstrap-guile (default-guile))
     #:options (list #:references-graphs `(("profile" ,profile))
                     #:target target)))