bug#64317: [Cuirass] Download products can disappear, leading to HTTP 500

[Ludovic Courtès]

The /download endpoint refers directly to store items.  However, they
can be GC’d in the meantime, in which case the HTTP handler fails
gracelessly (HTTP 500 or so) as shown here:

--8<---------------cut here---------------start------------->8---
2023-06-27 16:02:24 GET /download/718
2023-06-27 16:02:24 Uncaught exception in task:
2023-06-27 16:02:24 In fibers.scm:
2023-06-27 16:02:24     172:8  1 (_)
2023-06-27 16:02:24 In ice-9/boot-9.scm:
2023-06-27 16:02:24   1685:16  0 (raise-exception _ #:continuable? _)
2023-06-27 16:02:24 ice-9/boot-9.scm:1685:16: In procedure raise-exception:
2023-06-27 16:02:24 In procedure open-file: No such file or directory: 
"/gnu/store/bnsh1yk4l913af8swqi3x7xp1xsa8gq7-guix-binary.tar.xz"
--8<---------------cut here---------------end--------------->8---

The solution IMO is to (1) register GC roots for these, (2) periodically
remove old GC roots, and (3) return 404 when the file has vanished.

Ludo’.