fix(path): use os.getenv instead of vim.env in PathLib:expand

[napisani]

• PathLib:expand (and therefore PathLib:absolute) read environment variables via vim.env[env_var], whose __index is backed by
  Vimscript's getenv() — a call that requires Neovim's main/textlock context.
• PathLib:absolute is diffview's general-purpose path-resolution helper and gets called while processing paths returned from git
  subprocess output, which runs inside libuv job/process callbacks (fast-event context). On Neovim 0.10+ this throws an E5560-class
  error ("Vimscript function must not be called in a lua loop callback") for any path containing a literal $VAR segment.
• Replaced vim.env[env_var] with os.getenv(env_var) — a plain Lua/C call with no dependency on the main loop, safe from any context,
  and a drop-in equivalent for this read-only lookup.

[dlyongemallo]

Thanks @napisani for catching this.