Install of keybase_amd64.deb on Ubuntu 20.04 Focal Fossa ISO boot fails

For some reason, when booting the Ubuntu 20.04 Focal Fossa ISO and selecting "Try Ubuntu", installation of Keybase fails. Doing so on an installed system running Ubuntu 20.04 Focal Fossa works fine.

# Steps to reproduce

1. Boot a Ubuntu 20.04 ISO
2. Select "Try Ubuntu"
3. Open a Terminal
4. `sudo apt install curl`
5. `curl --remote-name https://prerelease.keybase.io/keybase_amd64.deb`
6. `sudo apt install ./keybase_amd64.deb`

# Expected results

Keybase would install on a stock Ubuntu LTS ISO

# Actual results

```
ubuntu@ubuntu:~$ curl --remote-name https://prerelease.keybase.io/keybase_amd64.deb
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 123M 100 123M 0 0 5617k 0 0:00:22 0:00:22 --:--:-- 5899k
ubuntu@ubuntu:~$ sudo apt install ./keybase_amd64.deb
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'keybase' instead of './keybase_amd64.deb'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
keybase : Depends: libappindicator1 but it is not installable
Depends: libgconf-2-4 but it is not installable
E: Unable to correct problems, you have held broken packages.
```

# What it looks like on an installed Ubuntu 20.04 system for comparison

```
user@myhost:~$ sudo apt install ./keybase_amd64.deb
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'keybase' instead of './keybase_amd64.deb'
The following additional packages will be installed:
gconf-service gconf-service-backend gconf2-common libappindicator1 libdbusmenu-gtk4 libgconf-2-4
Suggested packages:
indicator-application
The following NEW packages will be installed:
gconf-service gconf-service-backend gconf2-common keybase libappindicator1 libdbusmenu-gtk4 libgconf-2-4
0 upgraded, 7 newly installed, 0 to remove and 4 not upgraded.
Need to get 909 kB/130 MB of archives.
After this operation, 483 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
```

Reproduce the failure by booting the Ubuntu 20.04 ISO and following the listed curl and apt commands. Compare the keybase_amd64.deb dependency resolution with an installed Ubuntu 20.04 system, focusing on libappindicator1 and libgconf-2-4; done means the package installs successfully on the stock ISO.

Written by the indexing model from the issue text.