Guix is failing to build on core-updates for x86_64-linux:

https://hydra.gnu.org/job/gnu/core-updates/guix-0.12.0-5.1162.x86_64-linux

I can't reproduce this failure on my own machines.

Here is the backtrace, which has now been reproduced 3 times on the
build machines:

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gnu/build/vm.scm:317:4: warning: possibly unbound variable `umount'
Backtrace:
In ice-9/boot-9.scm:
 160: 9 [catch #t #<catch-closure 8c7760> ...]
In unknown file:
   ?: 8 [apply-smob/1 #<catch-closure 8c7760>]
In ice-9/boot-9.scm:
  66: 7 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 6 [eval # #]
In ice-9/boot-9.scm:
2412: 5 [save-module-excursion #<procedure 9027c0 at ice-9/boot-9.scm:4084:3 ()>]
4091: 4 [#<procedure 9027c0 at ice-9/boot-9.scm:4084:3 ()>]
1734: 3 [%start-stack load-stack #<procedure 914580 at ice-9/boot-9.scm:4080:10 ()>]
1739: 2 [#<procedure 9156c0 ()>]
In unknown file:
   ?: 1 [primitive-load "/tmp/guix-build-guix-0.12.0-5.1162.drv-0/source/./build-aux/compile-all.scm"]
In ice-9/threads.scm:
  99: 0 [loop (("guix/base32.scm" "guix/base64.scm" "guix/cpio.scm" ...))]

ice-9/threads.scm:99:22: In procedure loop:
ice-9/threads.scm:99:22: In procedure fport_write: Bad address
make[2]: *** [Makefile:4926: make-go] Error 1
make[2]: Leaving directory '/tmp/guix-build-guix-0.12.0-5.1162.drv-0/source'
make[1]: *** [Makefile:4069: all-recursive] Error 1
make[1]: Leaving directory '/tmp/guix-build-guix-0.12.0-5.1162.drv-0/source'
make: *** [Makefile:2725: all] Error 2
phase `build' failed after 133.0 seconds
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There is also some interesting garbled text earlier in the logs.