Use mksquashfs pseudofile definitions for AppImage build

[lalten]

This changes the way that appimage squashfs files are built with rules_appimage. Instead of taring up all the files and then providing that to mksquashfs we build a pseudo file definitions list that tells mksquashfs where to get all the files from. This avoids the expensive tar operation when inputs change (building the pseudofile defs is pretty cheap).
The mksquashfs action does take longer now (likely because it has to do thousands of sh -c "cat {file}" invocations) but overall the build times are lower for large appimages and the cachability remote execution ability should be better.

There was also an issue with the most recent MkAppDir implementation where the tar was populated in a ThreadPool which was not deterministic, so file order inside the .tar and .sqfs may change, leading to changed artifact hashes. This is resolved with this.