400_restore_duplicity.sh does unconditioned 'export HOSTNAME=$(hostname)'

[jsmeix]

• ReaR version ("/usr/sbin/rear -V"):
  current master code

• Brief description of the issue:
  In restore/DUPLICITY/default/400_restore_duplicity.sh there is

if [ "$BACKUP_PROG" = "duplicity" ]; then
...
    export HOSTNAME=$(hostname)

which means if at this point the HOSTNAME is not what hostname outputs
then the current HOSTNAME value is changed to what hostname outputs
not only while 400_restore_duplicity.sh runs but
also for all subsequent scripts that run afterwards.

This conflicts with how the HOSTNAME variable is meant to be used in ReaR.

By default he HOSTNAME value is set in default.conf as

HOSTNAME="$( hostname -s 2>/dev/null || uname -n | cut -d. -f1 )"

which could be different compared to HOSTNAME=$(hostname).

Additionally the HOSTNAME value can be specified different by the user
as he likes or needs in his particular case in his etc/rear/local.conf

• Work-around, if any:
  Use a different Duplicity specific variable name.

[jsmeix]

@Lukey3332
could you have a look here because you use Duplicity?

[jsmeix]

According to
git log -p --follow usr/share/rear/restore/DUPLICITY/default/400_restore_duplicity.sh
the HOSTNAME=$(hostname) exists since the beginning via
992df3e

[jsmeix]

With #1880 merged
this issue should be fixed.