bug#39685: Error on guix-pull using the qemu image

[Eilene Tomkins-Flanagan]

I downloaded the qemu image from https://ftp.gnu.org/gnu/guix/guix-system-vm-image-1.0.1.x86_64-linux.xz. It had a valid signature.

After booting the machine with qemu 3.1 (I'm on debian-amber):

`qemu-system-x86_64    -net user -net nic,model=virtio    -enable-kvm -m 512    -device virtio-blk,drive=myhd    -drive if=none,file=./guix-system-vm-image-1.0.1.x86_64-linux,id=myhd`

I tried installing hello for the guest user. It worked fine.

I then tried running `guix pull && guix package -u`, and received the following error:

```

[...]

substitute: updating substitutes from 'https://ci.guix.gnu.org'...100.0%

Backtrace:

          1 (apply-smob/1 #<catch-closure d6f220 quit)

          0 (apply-smob/1 #<catch-closure 6a2bbc0>)

 

ERROR: In procedure apply-smob/1

In procedure fport_write: Broken pipe

```

On the second attempt, it failed immediately with this message:

```

Updating channel 'guix' from Git repository at 'https://git.savannah.gnu.org/git/guix.git'...

Building from this channel:

  guix      https://guix.savannah.gnu.org/git/guix.git 6dde98c

Computing Guix derivation for 'x86_64-linux'.../guix pull: error: You found a bug: the program '/gnu/store/mr5q5qcwinaxy7f4cyfm08s05aj9rr6-compute-guix-derivation'

failed to compute the derivation for Guix (version: "6dde98c3d9e7a0385da57abedd9b41ca733acfc6"; system: "x86_64-linux";

host version: "1.0.1-1.8204295"; pull-version: 1).

Please report it by email to <bug-guix.gnu.org>

```

After asking on the IRC to find out if I'd made a mistake, they directed me to send in the error as the message said.

Let me know if other info is necessary for reproduction.

-Eilene