bug#18115: offload fails to register GC root

[Mark H Weaver <mhw@netris.org>]

See the aborted build here: http://hydra.gnu.org/build/80150

Here's the Nix error output:

--8<---------------cut here---------------start------------->8---
these derivations will be built:
  /gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv
process 11125 acquired build slot '/nix/var/guix/offload/chapters.gnu.org/0'
process 11125 acquired build slot '/nix/var/guix/offload/hydra.gnunet.org/0'
load on machine 'hydra.gnunet.org' is 0.05 (normalized: 0.0125)
load on machine 'chapters.gnu.org' is 0.01 (normalized: 0.005)
load on machine 'hydra.gnunet.org' is 0.05 (normalized: 0.0125)
@ build-started /gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv - x86_64-linux /nix/var/log/guix/drvs/jb//3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv
Backtrace:
In ice-9/boot-9.scm:
 157: 7 [catch #t #<catch-closure 25bd2e0> ...]
In unknown file:
   ?: 6 [apply-smob/1 #<catch-closure 25bd2e0>]
In ice-9/boot-9.scm:
  63: 5 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 4 [eval # #]
In unknown file:
   ?: 3 [call-with-input-string "(begin (use-modules (guix config)) (let ((root-directory (string-append %state-directory \"/gcroots/tmp\"))) (false-if-exception (mkdir root-directory)) (symlink \"/gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv\" (string-append root-directory \"/\" \"offload-20121227-hydra.gnu.org-11125\"))))" ...]
In ice-9/command-line.scm:
 180: 2 [#<procedure 25d6cc0 at ice-9/command-line.scm:175:6 (port)> #<input: string 239a5b0>]
In unknown file:
   ?: 1 [eval (begin (use-modules #) (let # # #)) #<directory (guile-user) 25b1c60>]
   ?: 0 [symlink "/gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv" ...]

ERROR: In procedure symlink:
ERROR: In procedure symlink: File exists
guix offload: error: failed to register GC root for '/gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv' on '#<<build-machine> name: "hydra.gnunet.org" port: 22 system: "x86_64-linux" user: "hydra" private-key: "/home/hydra/.lsh/identity" parallel-builds: 4 speed: 1.7 features: ()>' (status: 256)
builder for `/gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv' failed with exit code 1
@ hook-failed /gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv - 256 builder for `/gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv' failed with exit code 1
error: build of `/gnu/store/jb3wfz0barwb1f959l1ggi75zybs8ayh-gnumeric-1.12.17.drv' failed
--8<---------------cut here---------------end--------------->8---