fix(vim.loader): randomized AppImage path pollutes luac cache #35636

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 78bbe53f76)

runtime/lua/vim/loader.lua

@@ -7,6 +7,7 @@ local loaders = package.loaders
 local _loadfile = loadfile
 
 local VERSION = 4
+local is_appimage = (os.getenv('APPIMAGE') ~= nil)
 
 local M = {}
 
@@ -78,7 +79,15 @@ local function fs_stat_cached(path)
 end
 
 local function normalize(path)
-  return fs.normalize(path, { expand_env = false, _fast = true })
+  path = fs.normalize(path, { expand_env = false, _fast = true })
+
+  if is_appimage then
+    -- Avoid cache pollution caused by AppImage randomizing the program root. #31165
+    -- "/tmp/.mount_nvimAmpHPH/usr/share/nvim/runtime" => "/usr/share/nvim/runtime"
+    path = path:match('(/usr/.*)') or path
+  end
+
+  return path
 end
 
 local rtp_cached = {} --- @type string[]