Steps to build an ISO locally
The first step is to use livecd-rootfs to build artifacts

# create a bionic build environment
$ multipass launch bionic -n isobuilder --disk 50G --mem 3G #15G for livecd-rootfs, 35G for a full build
$ multipass shell isobuilder
$ git clone https://github.com/chrisglass/ubuntu-old-fashioned
$ cd ubuntu-old-fashioned
$ ./setup-old-fashioned
-> select the serie, ubuntu/master for impish builds
$ cd ~/livecd-rootfs
# apply fix that hasn't been merged yet
$ sudo -E old-fashioned-image-build --series impish --project ubuntu --subproject canary --image-format squashfs --use-chroot-cache

Optional set up of a local squid deb proxy
$ sudo apt install squid-deb-proxy
$ export LXD_ADDRESS=$(ifconfig lxdbr0 | grep 'inet' | cut -d: -f2 | awk '{ print $2}')
$ sudo -E bash -c 'cat > /etc/apt/apt.conf.d/30squid-proxy << EOF
Acquire::http::Proxy "http://$LXD_ADDRESS:8000";
EOF'
$ sudo editor /etc/squid-deb-proxy/autogenerated/allowed-networks-src.acl
Add the container IP to the list


The next step is to use ubuntu-cdimage / debian-cd / live-build to use the generated artifacts and turn them into an ISO

= hackish but working =

$ export CDIMAGE_NOSYNC=1
$ export CDIMAGE_ROOT=/home/ubuntu/cdimage/
$ export PATH=/home/ubuntu/cdimage/bin:$PATH

$ cd ~
$ bzr branch https://code.launchpad.net/~ubuntu-cdimage/ubuntu-cdimage/mainline cdimage
$ cd cdimage
$ bzr branch lp:~ubuntu-cdimage/debian-cd/ubuntu debian-cd
$ sudo apt install procmail python-apt rename liblz4-tool xorriso genisoimage
$ mkdir -p ftp/dists/impish
$ cd ftp/dists
$ rsync --recursive --links --perms --times --compress --progress --delete rsync://archive.ubuntu.com/ubuntu/dists/impish/ impish
$ rsync --recursive --links --perms --times --compress --progress --delete rsync://archive.ubuntu.com/ubuntu/dists/impish-updates/ impish-updates
$ cd ~/cdimage
$ git clone https://git.launchpad.net/germinate
$ mkdir -p ftp/pool/main/u/ubuntu-keyring
$ wget http://archive.ubuntu.com/ubuntu/pool/main/u/ubuntu-keyring/ubuntu-keyring_2021.03.26_all.deb -O/home/ubuntu/cdimage/ftp/pool/main/u/ubuntu-keyring/ubuntu-keyring_2021.03.26_all.deb
$ apply the hackish patch there

=== modified file 'lib/cdimage/livefs.py'
--- lib/cdimage/livefs.py       2020-12-03 21:38:05 +0000
+++ lib/cdimage/livefs.py       2020-12-14 20:48:18 +0000
@@ -660,6 +660,7 @@
 
 
 def download_live_items(config, arch, item):
+    return True
     output_dir = live_output_directory(config)
     found = False
 
@@ -789,6 +790,8 @@
     output_dir = live_output_directory(config)
     osextras.mkemptydir(output_dir)
 
+    os.system("cp /home/ubuntu/build.output/* %s" % output_dir)
+    os.system("rename s/livecd.ubuntu/amd64/ %s/*  " % output_dir)
     if (config["CDIMAGE_LIVE"] or config["CDIMAGE_SQUASHFS_BASE"] or
             config["CDIMAGE_PREINSTALLED"]):
         got_image = False

$ SUBPROJECT=canary DIST=impish PROJECT=ubuntu DEBUG=1 for-project ubuntu cron.daily-live 2> ftp/pool/deblist
$ cd ftp/pool
$ grep 'deb failed: No' deblist | awk '{print $3}' | sed 's#.*pool/##' | xargs -n1 dirname | sort | uniq | xargs -n1 mkdir -p
$ grep 'deb failed: No' deblist | awk '{print $3}' | sed 's#.*pool/##' | xargs -I@ wget "http://archive.ubuntu.com/ubuntu/pool/"@ -O@
$ cd ~/cdimage
$ mkdir -p /home/ubuntu/cdimage/ftp/pool/main/c/cd-boot-images-amd64/
$ wget http://archive.ubuntu.com/ubuntu/pool/main/c/cd-boot-images-amd64/cd-boot-images-amd64_14_all.deb -O/home/ubuntu/cdimage/ftp/pool/main/c/cd-boot-images-amd64/cd-boot-images-amd64_14_all.deb
(check the current version) 
$ mkdir -p /home/ubuntu/cdimage/ftp/pool/main/m/memtest86+/
$ wget http://archive.ubuntu.com/ubuntu/pool/main/m/memtest86+/memtest86+_5.01-3.1ubuntu4_amd64.deb -O/home/ubuntu/cdimage/ftp/pool/main/m/memtest86+/memtest86+_5.01-3.1ubuntu4_amd64.deb

On bionic needs to apply the fix from http://launchpadlibrarian.net/544367331/initramfs-tools_0.136ubuntu6.5_0.136ubuntu6.6.diff.gz
(was https://bugs.launchpad.net/ubuntu/+source/initramfs-tools/+bug/1832108 for < impish) (and install zstd!)

$ SUBPROJECT=canary DIST=impish PROJECT=ubuntu DEBUG=1 for-project ubuntu cron.daily-live

The image should be available as
scratch/ubuntu/impish/daily-live/debian-cd/amd64/impish-desktop-canary-amd64.raw