bug#19771: Missing documentation on enviroment variables for guix-deamon

[Andrei Osipov]

Consider adding the descrition of env variables used by guix-deamon to
the documentation. 

<andreoss`> how to change the temporary directory from /tmp to /var/tmp?
                                                                        [08:14]
<andreoss`> i have /tmp as tmpfs and it has no enough space to build gcc
<Sleep_Walker> hm, I can't find it either in ./configure, either as parameter
               to guix-daemon                                           [08:29]
<Sleep_Walker> it seems it could respect TMPDIR environment variable    [08:34]
<Sleep_Walker> andreoss`: could you confirm?                            [08:36]
<Sleep_Walker> export TMPDIR=/var/tmp ; guix-daemon <your_parameters>
<andreoss`> Sleep_Walker: confirmed                                     [08:39]
<andreoss`> thanks
<Sleep_Walker> andreoss`: you're welcome                                [08:42]
<Sleep_Walker> andreoss`: could you please file a bug and request
               documentation improvement?
<Sleep_Walker> so it doesn't get lost