services: Add SRFI-26 to Nix activation gexp.

Fix the following fatal error on ‘guix system reconfigure’:

Backtrace:
          18 (primitive-load "/home/nckx/.config/guix/current/bin/guix")
In guix/ui.scm:
  1620:12 17 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    829:9 16 (catch _ _ #<procedure 7a7caa2d4c80 at guix/ui.scm:587:2 (key c)> _)
    829:9 15 (catch _ _ #<procedure 7a7caa2d4c98 at guix/ui.scm:710:6 (key proc format-string format-args . …> …)
In guix/scripts/system.scm:
   1325:8 14 (_)
In guix/status.scm:
    768:4 13 (call-with-status-report _ _)
In guix/scripts/system.scm:
   1181:4 12 (process-action _ _ _)
In guix/store.scm:
   623:10 11 (call-with-store _)
  1800:24 10 (run-with-store _ _ #:guile-for-build _ #:system _ #:target _)
In guix/scripts/system.scm:
   920:13  9 (_ _)
    409:8  8 (_ _)
In unknown file:
           7 (primitive-load "/gnu/store/dha7j9gcz3vgb8cy7vfvvaswwsywrrzj-activate")
In ice-9/boot-9.scm:
   260:13  6 (for-each #<procedure primitive-load (_)> _)
In unknown file:
           5 (primitive-load "/gnu/store/ql1xjxrssxm51z2bn2v6l3mlrncij84h-activate-service")
In ice-9/eval.scm:
    619:8  4 (_ #f)
   196:35  3 (_ #f)
   196:27  2 (_ #f)
   223:20  1 (proc #<directory (guile-user) 1fa4140>)
In unknown file:
           0 (%resolve-variable (7 . cut) #<directory (guile-user) 1fa4140>)

ERROR: In procedure %resolve-variable:
error: cut: unbound variable

Reported-by: Alex McGrath on #guix.

* gnu/services/nix.scm (nix-activation): Import and use (srfi srfi-26).

gnu/services/nix.scm

@@ -75,9 +75,11 @@ GID."
 
 (define (nix-activation _)
   "Return the activation gexp."
-  (with-imported-modules '((guix build utils))
+  (with-imported-modules '((guix build utils)
+                           (srfi srfi-26))
     #~(begin
-        (use-modules (guix build utils))
+        (use-modules (guix build utils)
+                     (srfi srfi-26))
         (for-each (cut mkdir-p <>) '("/nix/store" "/nix/var/log"
                                      "/nix/var/nix/gcroots/per-user"
                                      "/nix/var/nix/profiles/per-user"))