bug#57095: another "inappropriate ioctl" bug :)

[Csepp]

```
./pre-inst-env guix import pypi -r linode-cli | tee -a 
gnu/packages/python-xyz.scm

...
In guix/build/syscalls.scm:
  2284:35  1 (_)
   2273:8  0 (terminal-window-size _)

guix/build/syscalls.scm:2273:8: In procedure terminal-window-size:
In procedure terminal-window-size: Inappropriate ioctl for device
```

I assume the progress bar code does not gracefully handle the very
common case when stdout is not a terminal.

Good thing I'm in a local checkout so I can just make it return some
constant.