Cannot install Keybase on PureOS

TL;dr: It would be so great if Keybase were on Flatpack

That said, I have installed Keybase on Linux before but the distro I have won't seem to do it. I think PureOS is Debian related but I'm not sure.

I tried all the versions of the instructions on the Linux installation page and the only one that got anywhere was Debian.

So, the first command seemed to go fine, but the second one fell down and I don't know what's wrong.
Here's what I got:
```:~$ 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
100 243 0 243 0 0 2430 0 --:--:-- --:--:-- --:--:-- 2405
bradydale@MisterDelay:~$ sudo apt install ./keybase_i386.deb
[sudo] password for bradydale:
Reading package lists... Done
E: Unsupported file ./keybase_i386.deb given on commandline```

Review the Linux installation page and the captured curl and apt commands, checking the downloaded filename against the package filename being installed. Done means the requested PureOS installation path or Flatpak availability is clearly supported or documented, with the reported failure addressed.

Written by the indexing model from the issue text.