test failed in CI: `/tmp` leftover detritus from unknown test (?)

This test failed on a CI run on #11058:

https://github.com/oxidecomputer/omicron/pull/11058/checks?check_run_id=93879397312

Log showing the specific test failure:

https://buildomat.eng.oxide.computer/wg/0/details/01KZS0CNV26CYWA5H7CQDTCA8G/eFlDh21hFYQqdGZpQ8tBnGqhri8TUi8KXLx8g0Qpyisgap9T/01KZS0D6S4D0MBQNYS6QWEM72Z

Excerpt from the log showing the failure:

No tests failed, but the final check that we didn't leave anything behind in /tmp failed:

12742	2026-08-11T18:41:35.981Z	files in /var/tmp/omicron_tmp (none expected on success):
12743	2026-08-11T18:41:35.981Z	+ find /var/tmp/omicron_tmp -ls
12744	2026-08-11T18:41:35.981Z	   258335     24 drwxr-xr-x   4 build    build       20480 Aug 11 18:41 /var/tmp/omicron_tmp
12745	2026-08-11T18:41:35.981Z	  1555522      4 drwxr-xr-x   2 build    build        4096 Aug 11 18:37 /var/tmp/omicron_tmp/.tmpiWcGzh
12746	2026-08-11T18:41:35.981Z	  6967634      4 -rw-r--r--   1 build    build         728 Aug 11 18:37 /var/tmp/omicron_tmp/.tmpiWcGzh/3adabc83af46f4c5c09e9f94ca9a80de4a8b9ba8986d2827ea546fe2397eefce
12747	2026-08-11T18:41:35.981Z	  1555523      4 drwxr-xr-x   2 build    build        4096 Aug 11 18:37 /var/tmp/omicron_tmp/.tmpZyslXj
12748	2026-08-11T18:41:35.981Z	  6967720      4 -rw-r--r--   1 build    build         150 Aug 11 18:37 /var/tmp/omicron_tmp/.tmpZyslXj/addc4ed18936fcdae28bb14ec868930765bce4948be459599d7e9991ed24ee87

addc4ed18936fcdae28bb14ec868930765bce4948be459599d7e9991ed24ee87 is a .tar.gz containing a single oxide.json file:

% cat oxide.json
{"v":"1","t":"layer","pkg":"crucible-pantry-zone","version":"2.0.0"}

3adabc83af46f4c5c09e9f94ca9a80de4a8b9ba8986d2827ea546fe2397eefce is a .zip file containing a fake hubris image:

% unzip -l 3adabc83af46f4c5c09e9f94ca9a80de4a8b9ba8986d2827ea546fe2397eefce
Archive:  3adabc83af46f4c5c09e9f94ca9a80de4a8b9ba8986d2827ea546fe2397eefce
hubris build archive v0
  Length      Date    Time    Name
---------  ---------- -----   ----
     1248  1980-01-01 00:00   img/final.elf
     1024  1980-01-01 00:00   img/final.bin
---------                     -------
     2272                     2 files

I'd expect these files to be produced by various update- or TUF-related tests, but I have no idea why these were left behind without anything else specific to the test that created them.

Start with the linked CI log for pull request #11058 and inspect the update- or TUF-related tests that could create files under /var/tmp/omicron_tmp. Trace the cleanup path for the .tar.gz and .zip artifacts, then rerun the relevant tests and confirm the directory is empty after success.