test(tui_spec): remove unnecessary job_opts variable (#40334)

This was initially added so that the __NVIM_DETACH environment variable
can be added to the jobs started in this test. That environment variable
is no longer needed, and there is also vim.tbl_extend() that can be used
to add an environment variable to a job anyway.

Also, make a shallow copy of opts.env in setup_child_nvim(), as mutating
the opts.env passed in may mask problems in other tests.

(cherry picked from commit f876fd906e)

test/functional/testterm.lua

@@ -203,7 +203,7 @@ function M.setup_child_nvim(args, opts)
   opts = opts or {}
   local argv = { nvim_prog, unpack(args or {}) }
 
-  local env = opts.env or {}
+  local env = t.shallowcopy(opts.env) or {}
   env.VIMRUNTIME = env.VIMRUNTIME or os.getenv('VIMRUNTIME')
   env.NVIM_TEST = env.NVIM_TEST or os.getenv('NVIM_TEST')