[build_debian.sh] fix systemd is not from backports buster (#7323)

#### Why I did it
To fix systemd is older version.

#### How I did it
Install systemd explicitelly

#### How to verify it
systemd --version

build_debian.sh

@@ -349,6 +349,7 @@ sudo LANG=C chroot $FILESYSTEM_ROOT bash -c "find /usr/share/i18n/locales/ ! -na
 # more up-to-date (but potentially less stable) versions
 sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y -t $IMAGE_DISTRO-backports install \
     picocom \
+    systemd \
     systemd-sysv
 
 if [[ $CONFIGURED_ARCH == amd64 ]]; then