Update fpm for broken packaging in CI

[katef]

recently fpm started giving errors I don't understand, about some ruby-related require thing. I think it's the same bug reported in jordansissel/fpm#1716

I had been using bpicode/github-action-fpm to run fpm (which I think does it in a docker image). In this PR I'm removing that, and installing fpm by hand instead. This way I can better see what's happening I guess. I don't know how to use docker or ruby, or do CI stuff really, so the fewer things the better for me.

fpm's docs on installing say it depends on:

apt-get install ruby ruby-dev rubygems build-essential

github's ubuntu environment provides these already.

This still gives the same warning:

fpm -v
Doing `require 'backports'` is deprecated and will not load any backport in the next major release.
Require just the needed backports instead, or 'backports/latest'.
1.11.0

so I have no idea what's happening here with the versions, but it works again for now, so hooray! Good job me.