ci: install signal in desktop vm and take screenshots

[jnsgruk]

This is a test that adds the ability to launch a KVM accelerated desktop VM inside the Github Actions runner.

Once running, we push the built snap into the VM and run signal-desktop. From there, we can use gnome-screenshot to get a screenshot of both the whole screen and the specific window we just launched.

Then finish by pulling the screenshot files out of the VM and adding them in a comment on the call for testing post.

[jnsgruk]

@merlijn-sebrechts it worked!

[jnsgruk]

@merlijn-sebrechts this is ready for review, I've tested the build all works and that the screenshots get uploaded, but obviously can't really test the code that happens after merge just yet!

I've given it a couple of reads, and I think the variables etc. are all wired up correctly, but we'll see when it merges!