Error building with guix system image

[Saku Laesvuori]

Running `guix system image image-minimal.scm` fails with the following
build log:

```build log for partition.img.drv
copying 340 store items | [progress bar animation]
registering 337 items | [progress bar animation, fails before end]
Backtrace:
In ice-9/eval.scm:
    619:8 19 (_ #(#<directory (guile-user) 7fffeffcfc80> #<proced…> …))
In ./gnu/build/image.scm:
    236:4 18 (initialize-root-partition "tmp-root" #:bootcfg _ # _ # …)
In ice-9/boot-9.scm:
   260:13 17 (for-each #<procedure 7fffed357b80 at ./gnu/build/imag…> …)
In ./gnu/build/image.scm:
    167:6 16 (register-closure "tmp-root" "system" #:schema _ # _)
In ./guix/store/database.scm:
   111:20 15 (call-with-database "tmp-root/var/guix/db/db.sqlite" # # …)
In ./guix/progress.scm:
    71:36 14 (call-with-progress-reporter #<<progress-reporter> sta…> …)
In srfi/srfi-1.scm:
    634:9 13 (for-each #<procedure 7fffec850d40 at ./guix/store/dat…> …)
In ./guix/store/database.scm:
   420:36 12 (_ #<<store-info> item: "/gnu/store/8z1q7vjxylm6l4ibsyw…>)
In ./guix/store/deduplication.scm:
     42:4 11 (nar-sha256 "tmp-root/gnu/store/8z1q7vjxylm6l4ibsywd4h6…")
In ./guix/serialization.scm:
    381:9 10 (dump "tmp-root/gnu/store/8z1q7vjxylm6l4ibsywd4h6m6jv6z…")
In srfi/srfi-1.scm:
    634:9  9 (for-each #<procedure 7fffed493990 at ./guix/serializa…> …)
In ./guix/serialization.scm:
   386:23  8 (_ "share")
    381:9  7 (dump "tmp-root/gnu/store/8z1q7vjxylm6l4ibsywd4h6m6jv6z…")
In srfi/srfi-1.scm:
    634:9  6 (for-each #<procedure 7fffed4936f0 at ./guix/serializa…> …)
In ./guix/serialization.scm:
   386:23  5 (_ "locale")
    381:9  4 (dump "tmp-root/gnu/store/8z1q7vjxylm6l4ibsywd4h6m6jv6z…")
In srfi/srfi-1.scm:
    634:9  3 (for-each #<procedure 7fffed4933f0 at ./guix/serializa…> …)
In ./guix/serialization.scm:
   386:23  2 (_ "ja")
   380:43  1 (dump "tmp-root/gnu/store/8z1q7vjxylm6l4ibsywd4h6m6jv6z…")
In srfi/srfi-1.scm:
   691:29  0 (filter-map #<procedure 7fffeea6d9e0 at ./guix/seriali…> …)

srfi/srfi-1.scm:691:29: In procedure filter-map:
In procedure car: Wrong type argument in position 1 (expecting pair): #f
environment variable `PATH' set to 
`/gnu/store/zfiwdkwfs039q346nkynvhllxbra2rg8-e2fsprogs-1.46.4/bin:/gnu/store/zfiwdkwfs039q346nkynvhllxbra2rg8-e2fsprogs-1.46.4/sbin:/gnu/store/nj43wfbldxrwfmrk9vgvliz28iyl89dp-fakeroot-1.30.1/bin:/gnu/store/lpkpldxni8vab82kwq9qs032hni580ak-dosfstools-4.2/sbin:/gnu/store/131xfb6iz9vh7c8s7kv6nhvycrqcfad8-mtools-4.0.42/bin'
```

Does anyone have any pointers on how to debug or fix this? The operating
system builds nicely with `guix system vm`. Building
`gnu/system/examples/bare-bones.tmpl` with `guix system image` also
fails with a similar error and works correctly with `guix system vm`.


```image-minimal.scm
(use-modules
  (gnu)
  (gnu system image))

(define %operating-system
  (operating-system
   (host-name "image-test")
   (bootloader (bootloader-configuration
                (bootloader grub-bootloader)
                (targets '("/dev/sdX"))))
   (file-systems (cons*
                  (file-system
                   (mount-point "/")
                   (device "/dev/sdX1")
                   (type "ext4"))
                  %base-file-systems))))

(image-with-os (raw-with-offset-disk-image)
               %operating-system)
```

Output of `guix describe` just in case, though the error also occured
with the latest guix version when I tested.

```channels.scm
(list (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git";)
        (branch "master")
        (commit
          "5c893787be78a79433fe1343f5b70cd647e8f667")
        (introduction
          (make-channel-introduction
            "9edb3f66fd807b096b48283debdcddccfea34bad"
            (openpgp-fingerprint
              "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA")))))
```

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