Can an approach be helpful here? 
mkdir /var/snap/juju-db/common/tmp
sudo mount --bind /tmp /var/snap/juju-db/common/tmp

EDIT 

Unfortunately does not work 

root@juju-maas-2:~# ll /var/snap/juju-db/common/tmp/
total 761264
drwxrwxrwt 15 root root      4096 Dec 17 14:42 ./
drwx------  5 root root      4096 Dec 17 12:56 ../
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .ICE-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .Test-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .X11-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .XIM-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .font-unix/
drwxr-xr-x  4 root root      4096 Dec 16 23:40 bkp/
-rw-r--r--  1 root root 779465185 Dec 16 23:44 juju-controller.tgz
-rw-------  1 root root         0 Dec 13 09:35 juju-machine-lock
drwx------  4 root root      4096 Dec 13 09:31 snap-private-tmp/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-ModemManager.service-I2Tlzz/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-systemd-logind.service-YycNbR/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-systemd-resolved.service-6GvnaA/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-systemd-timesyncd.service-wsZS07/
drwx------  3 root root      4096 Dec 13 17:06 systemd-private-c45b760838cb4927870de045cc36ddc2-upower.service-SChbOO/
drwx------  2 root root      4096 Dec 13 09:35 tmux-0/
root@juju-maas-2:~# ll /tmp
total 761264
drwxrwxrwt 15 root root      4096 Dec 17 14:42 ./
drwxr-xr-x 19 root root      4096 Dec 13 09:31 ../
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .ICE-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .Test-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .X11-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .XIM-unix/
drwxrwxrwt  2 root root      4096 Dec 13 09:31 .font-unix/
drwxr-xr-x  4 root root      4096 Dec 16 23:40 bkp/
-rw-r--r--  1 root root 779465185 Dec 16 23:44 juju-controller.tgz
-rw-------  1 root root         0 Dec 13 09:35 juju-machine-lock
drwx------  4 root root      4096 Dec 13 09:31 snap-private-tmp/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-ModemManager.service-I2Tlzz/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-systemd-logind.service-YycNbR/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-systemd-resolved.service-6GvnaA/
drwx------  3 root root      4096 Dec 13 09:31 systemd-private-c45b760838cb4927870de045cc36ddc2-systemd-timesyncd.service-wsZS07/
drwx------  3 root root      4096 Dec 13 17:06 systemd-private-c45b760838cb4927870de045cc36ddc2-upower.service-SChbOO/
drwx------  2 root root      4096 Dec 13 09:35 tmux-0/
root@juju-maas-2:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           364M  1.3M  363M   1% /run
/dev/sda2        18G   13G  4.7G  73% /
tmpfs           1.8G     0  1.8G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs            50M   13M   38M  25% /run/lxd_agent
/dev/sda1       511M  5.3M  506M   2% /boot/efi
tmpfs           364M  4.0K  364M   1% /run/user/0
root@juju-maas-2:~# moount |grep tmp
Command 'moount' not found, did you mean:
command 'mmount' from deb mtools (4.0.33-1+really4.0.32-1build1)
command 'mount' from deb mount (2.37.2-4ubuntu3)
Try: apt install <deb name>
root@juju-maas-2:~# mount |grep tmp
udev on /dev type devtmpfs (rw,nosuid,relatime,size=1805892k,nr_inodes=451473,mode=755,inode64)
tmpfs on /run type tmpfs (rw,nosuid,nodev,noexec,relatime,size=372544k,mode=755,inode64)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev,inode64)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k,inode64)
tmpfs on /run/lxd_agent type tmpfs (rw,relatime,size=51200k,mode=700,inode64)
tmpfs on /run/snapd/ns type tmpfs (rw,nosuid,nodev,noexec,relatime,size=372544k,mode=755,inode64)
/dev/sda2 on /var/snap/juju-db/common/tmp type ext4 (rw,relatime)
tmpfs on /run/user/0 type tmpfs (rw,nosuid,nodev,relatime,size=372540k,nr_inodes=93135,mode=700,inode64)
root@juju-maas-2:~#