Use tempfile::TempDir::keep() instead of deprecated into_path()

[emmanuel-ferdman]

PR Summary

The tempfile library has deprecated TempDir::into_path(), replacing it by TempDir::keep() starting version 3.3. It leads to the following warnings which you can find in the CI logs:

warning: use of deprecated method `tempfile::TempDir::into_path`: use TempDir::keep()
   --> src/lib.rs:211:38
    |
211 |             let temp_file = temp_dir.into_path().join("result.asm");

[antoyo]

Thanks for your contribution!