Hi Guix!

I just noticed an issue building the website: with a recent change, the
code would check explicitely for GUIX_WEB_SITE_LOCAL in the
environment, but .guix.scm doesn't set it, so it fails with:

;;; (repl-version (repl-version 0 1 1))
Running 'haunt build' for lingua de_DE...
Backtrace:
In haunt/ui.scm:
    125:6 19 (run-haunt-command _ . _)
In haunt/ui/build.scm:
     60:4 18 (haunt-build . _)
In haunt/site.scm:
   130:14 17 (build-site _)
In haunt/utils.scm:
    64:11 16 (flat-map _ . _)
In srfi/srfi-1.scm:
   586:29 15 (map1 _)
   586:29 14 (map1 _)
   586:29 13 (map1 _)
   586:29 12 (map1 _)
   586:17 11 (map1 (#<procedure 7fffed0befc0 at ice-9/boot-9.scm:…> …))
In unknown file:
          10 (_ #<procedure 7fffeb2950a0 at ice-9/boot-9.scm:798:28…> …)
In ice-9/eval.scm:
    155:9  9 (_ _)
    155:9  8 (_ _)
   173:39  7 (_ #(#(#<directory (apps packages builder) 7ffff49b…>) …))
   293:34  6 (_ #(#(#(#<directory (apps packages builder) 7ffff…>)) …))
    159:9  5 (_ #(#(#(#<directory (apps packages builder) 7ffff…>)) …))
    159:9  4 (_ _)
   196:43  3 (_ _)
In unknown file:
           2 (force #<promise #<procedure 7fffed0f3480 at ice-9/eval…>)
In ice-9/eval.scm:
   245:16  1 (_ #(#(#(#<directory (apps packages data) 7ffff49b…>)) …))
In unknown file:
           0 (string=? "yes" #f)

ERROR: In procedure string=?:
In procedure string=: Wrong type argument in position 2 (expecting
string): #f building pages in '/tmp/gnu.org/software/guix'...
Backtrace:
           3 (primitive-load "/gnu/store/8amiibdjbp3chrj8cnbg6v1162g?")
In ice-9/boot-9.scm:
   260:13  2 (for-each #<procedure 7ffff5fb1140 at ice-9/eval.scm:3?> ?)
In ice-9/eval.scm:
    619:8  1 (_ #(#(#<directory (guile-user) 7ffff5fefc80>) "de_DE"))
In guix/build/utils.scm:
    654:6  0 (invoke _ . _)

guix/build/utils.scm:654:6: In procedure invoke:
ERROR:
  1. &invoke-error:
      program:
"/gnu/store/2p1zk377cvxs1dz0iasj2l5wp2jlpcx2-haunt-0.2.5/bin/haunt"
arguments: ("build") exit-status: 1
      term-signal: #f
      stop-signal: #f

The attached patch fixes the issue.