bug#43011: unattended-upgrade fails and doesn't log why

[Jesse Gibbons]

An example is the attached log. It says the process failed with status 
1, but it doesn't say what failed or even reference a log in 
/var/log/guix. The only way to find out what could have caused it is to 
run the command, which might not fail the second time if it was a 
network error or (IIUC) if the issue was fixed between failure and retry.
[2020-08-23T01:30:00-0600] starting upgrade...
command "/gnu/store/a3qjvkp2z4h96ycc9afir5yrqbbsixbf-guix-1.1.0-18.218a67d/bin/guix" "time-machine" "-C" "/gnu/store/qkccph0zyh7nmwab6hisbhya7agsfb3y-channels.scm" "--" "system" "reconfigure" "/run/current-system/configuration.scm" failed with status 1