`modules/background_task/task.nu`: can not use a module inside closure in current directory.

Given tmp.nu in home directory, or any other directory except /tmp
I cannot run task spawn with the following closure:

task spawn { print $env.PWD; use tmp.nu }

When I run task log, it reports the following error message:

Error: nu::parser::module_not_found

  x Module not found.
   ,-[/tmp/tmp.Id6nNnlTch-nu-task:1:22]
 1 |  print $env.PWD; use tmp.nu
   :                      ^^^|^^
   :                         `-- module tmp.nu not found
   `----
  help: module files and their paths must be available before your script is
        run as parsing occurs before anything is evaluated

I believe it's because task spawn save the closure into a temporary file, which makes nushell doesn't try to load module from current directory. I don't think it's a nushell issue, maybe we can just change task.nu to fix it.

Start in modules/background_task/task.nu and reproduce the reported task spawn closure with tmp.nu in the current directory, then inspect how the closure is saved and run. Done means task spawn can use a module from the current directory and task log no longer reports module_not_found.