Tempfiles exported for use by a lua script (e.g. `hugin`) disappear immediately after export

Steps to reproduce:

1. enable contrib/hugin script, and ensure paths are configured correctly and hugin is installed
2. select images you want to stitch into a panorama
3. in export dialog, select "hugin panorama" as "target storage"
4. select file format (e.g. TIFF)
5. (optionally, makes no difference) tick "launch hugin gui"
6. run inotifywait -m /tmp -e create --format '%w%f' | while read f; echo "$f"; end (or similar) in terminal
7. press "start export"
8. wait for export to finish, see toast message "launching hugin"
9. nothing happens
10. check inotify and see that all images were created at one point (i.e. one log line per image in the form of /tmp/DSCF0721.tif)
11. check dartable log and see:

    sh: line 1: '/path/to/hugin' '/tmp/DSCF0720.tif' '/tmp/DSCF0713.tif' '/tmp/DSCF0721.tif' '/tmp/DSCF0715.tif' '/tmp/DSCF0718.tif' '/tmp/DSCF0717.tif' '/tmp/DSCF0716.tif' '/tmp/DSCF0714.tif' '/tmp/DSCF0719.tif' : No such file or directory
    

12. note: /path/to/hugin DOES exist and is the hugin executable
13. ls /tmp shows images are gone

I assume they somehow get cleaned from /tmp after the darktable export process (?) exits and the files are essentially orphaned.

Start with the contrib/hugin script and reproduce the export while watching /tmp and the darktable log. Trace when the exported images are cleaned up relative to the hugin launch; done means the generated images remain available when hugin is invoked and the reported export workflow opens successfully.