always use newest appimagetool version

[notimaginative]

This will download the appimagetool utility as part of the install step so that the newest version of both the tool and the runtime is always used. Moving it out of the build image allows us to take advantage of newer capabilities, such as no longer having an external dependency on libfuse2.

[notimaginative]

Forgot to comment here apparently, but this was originally part of the new image and workflow update. Since that's been delayed I split this change off, modified it to work with the existing build image, then generated a set of builds using the existing build setup and confirmed that the resulting AppImages work without fuse2 being installed on the system.

The downside of this change is that working internet is required to download appimagetool and a new type-2 runtime during the CI step. For our builds that shouldn't be a problem since working internet is already needed for the configure step (to get prebuilt libs). But it's something to be aware of.

General compatibility should be identical to existing FSO builds with the exception of no longer having an external dependency on libfuse2.

I'll hold off on merging in case someone else would like to comment.