Problem:
When using the Nvim appimage, `~/.cache/nvim/luac` directory can grow to
250,000+ files.

Example of 2 identical files in `./luac/`:

    %2ftmp%2f.mount_nvim.a65Rja0%2fusr%2fshare%2fnvim%2fruntime%2flua%2fvim%2ftreesitter.luac
    %2ftmp%2f.mount_nvim.aNpxXgo%2fusr%2fshare%2fnvim%2fruntime%2flua%2fvim%2ftreesitter.luac

Analysis:
The `nvim.appimage` mounts nvim at a different temporary path each time
it is invoked. The naming scheme of these cache files is random, which
defats the purpose of the cache creates N new files on every launch of
nvim.

Steps to reproduce:
1. install `nvim.appimage`
2. `mv ~/.cache/nvim/luac ~/.cache/nvim/luac.backup`
3. `nvim`
4. Observe contents of `~/.cache/nvim/luac/`
5. Close nvim and run `nvim` again
6. Observe contents of `~/.cache/nvim/luac/` and see that new identical
   files have been added with a different mount prefix

Solution:
When running from an appimage, trim the random part of the filepaths.

(cherry picked from commit 78bbe53)