casper interactive_netmount looks for the wrong url on 20.04.3

[Keno Fischer]

In https://git.launchpad.net/ubuntu/+source/casper/tree/scripts/casper?id=57382352df0fd500aeb132bfd3373c8463a7b46d#n385, casper attempts to construct the iso URL for an interactive_netmount as

```
            case $DPKG_ARCH in
                amd64)
                    server_url=https://releases.ubuntu.com/$UBUNTU_CODENAME/ubuntu-$VERSION_ID-live-server-$DPKG_ARCH.iso
                    desktop_url=https://releases.ubuntu.com/$UBUNTU_CODENAME/ubuntu-$VERSION_ID-desktop-$DPKG_ARCH.iso
                    ;;
                *)
                    server_url=http://cdimage.ubuntu.com/releases/$UBUNTU_CODENAME/release/ubuntu-$VERSION_ID-live-server-$DPKG_ARCH.iso
                    desktop_url=http://cdimage.ubuntu.com/releases/$UBUNTU_CODENAME/release/ubuntu-$VERSION_ID-desktop-$DPKG_ARCH.iso
                    ;;
```

However, on Ubuntu 20.04.3, /etc/os-release in the initrd reads:

```
NAME="Ubuntu"
VERSION="20.04.3 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.3 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
```

i.e. VERSION_ID is 20.04. For the above URL pattern to work, the VERSION_ID would have
to be 20.04.3. As a result, the interactive_netmount functionality fails with
a 404 on 20.04.3 unless the correct url is manually typed in.

[Łukasz Zemczak (sil2100)]

Ok, yes, we can get this semi-fixed for sure. It's basically been broken since start of focal - this has been something that Dimitri added in the 20.04 cycle and it always had this bug.

[Brian Murray (brian-murray)]

Given that this wasn't discovered until after multiple point releases of Ubuntu and that this installation method is a corner case we will not be prioritizing fixing this right now.

[Michael Hudson-Doyle (mwhudson)]

I'm also not sure how to solve this really, I don't think there's any way to know in the livefs build which release the initrd is going to end up being part of ...