bug#22676: (git) guix build failure "broken-tarball-fetch"

[Danny Milosavljevic]

Package: guix

In git guix (commit b042bdc8fdc2a89e51da852339c0ee86b7bedaf1) and recent GuixSD 
(that means I did guix pull after booting the tarball from the guix website 
about 5 days ago), I get a build failure when I do

$ ./pre-inst-env guix environment guix
$ make
$ touch gnu/packages/engineering.scm # note: file existed before.
$ make # will fail.
.

The error message is:

------
LOAD     (gnu packages engineering)
;;; note: source file ./gnu/packages/engineering.scm
;;;       newer than compiled /home/dannym/src/guix/gnu/packages/engineering.go
Backtrace:
In unknown file:
   ?: 19 [primitive-load-path "gnu/packages/version-control" ...]
In gnu/packages/version-control.scm:
  28: 18 [#<procedure 276d0a0 ()>]
In ice-9/boot-9.scm:
2951: 17 [define-module* (gnu packages version-control) #:filename ...]
2926: 16 [resolve-imports ((# # #) (#) (#) (#) ...)]
2864: 15 [resolve-interface (gnu packages emacs) #:select ...]
2789: 14 [#<procedure bc86e0 at ice-9/boot-9.scm:2777:4 (name #:optional 
autoload version #:key ensure)> # ...]
3065: 13 [try-module-autoload (gnu packages emacs) #f]
2401: 12 [save-module-excursion #<procedure 27e6900 at ice-9/boot-9.scm:3066:17 
()>]
3085: 11 [#<procedure 27e6900 at ice-9/boot-9.scm:3066:17 ()>]
In unknown file:
   ?: 10 [primitive-load-path "gnu/packages/emacs" ...]
In gnu/packages/emacs.scm:
  24: 9 [#<procedure 280a740 ()>]
In ice-9/boot-9.scm:
2951: 8 [define-module* (gnu packages emacs) #:filename ...]
2926: 7 [resolve-imports ((# # license:) (#) (#) (#) ...)]
2864: 6 [resolve-interface (gnu packages scheme) #:select ...]
2789: 5 [#<procedure bc86e0 at ice-9/boot-9.scm:2777:4 (name #:optional 
autoload version #:key ensure)> # ...]
3065: 4 [try-module-autoload (gnu packages scheme) #f]
2401: 3 [save-module-excursion #<procedure 27f02d0 at ice-9/boot-9.scm:3066:17 
()>]
3085: 2 [#<procedure 27f02d0 at ice-9/boot-9.scm:3066:17 ()>]
In unknown file:
   ?: 1 [primitive-load-path "gnu/packages/scheme" ...]
In gnu/packages/scheme.scm:
 493: 0 [#<procedure 2862860 ()>]

gnu/packages/scheme.scm:493:0: In procedure #<procedure 2862860 ()>:
gnu/packages/scheme.scm:493:0: In procedure module-lookup: Unbound variable: 
broken-tarball-fetch
------

Leo also reproduced the problem using the steps above.