shell: Create cached profile directory once connected to the daemon.

Fixes a regression introduced in d12c4452a4,
where ‘guix shell’, on a fresh installation, would attempt to create the
cached profile directory under /var/guix/profiles/per-user/$USER, which only
guix-daemon can do, on the user’s first connection.

* guix/scripts/shell.scm (options-with-caching): Remove ‘mkdir-p’ call for
ROOT’s parent directory.  Add ‘create-gc-root-directory?’ key to the result.
* guix/scripts/environment.scm (guix-environment*): Honor
‘create-gc-root-directory?’ key.

Fixes: guix/guix#126
Reported-by: Maxim Cournoyer <maxim@guixotic.coop>
Change-Id: I782461c80c254467108c6b8d676d269866795746

guix/scripts/environment.scm

@@ -1235,6 +1235,8 @@ command-line option processing with 'parse-command-line'."
                                       (if prof-drv (list prof-drv) '())
                                       (if (derivation? bash) (list bash) '()))))
                 (mwhen gc-root
+                  (mwhen (assoc-ref opts 'create-gc-root-directory?)
+                    (return (mkdir-p (dirname gc-root))))
                   (register-gc-root profile gc-root))
 
                 (mwhen (assoc-ref opts 'check?)

guix/scripts/shell.scm

@@ -261,10 +261,14 @@ cache) or a 'gc-root' key (to add the profile to cache)."
                               opts)))             ;load right away
         (if (and root (not (assq-ref opts 'gc-root)))
             (begin
-              (if stat
-                  (delete-file root)
-                  (mkdir-p (dirname root)))
-              (alist-cons 'gc-root root opts))
+              (when stat
+                (delete-file root))
+              ;; Note: Delay the creation of ROOT's parent directory: a
+              ;; connection must first be made to the daemon so that it
+              ;; creates /var/guix/profiles/per-user/$USER if needed.
+              (alist-cons 'gc-root root
+                          (alist-cons 'create-gc-root-directory? #t
+                                      opts)))
             opts))))
 
 (define (auto-detect-manifest opts)