Racy systemd-sysext-initrd.service on test 46 on Ubuntu

[bdrung]

Describe the bug

The test 46 is flaky on Ubuntu:

[    2.459257] systemd-sysext[150]: Failed to read metadata for image dracut-sysext: No such device
[FAILED] Failed to start systemd-sysext-ini…ension Images into /usr/ and /opt/.

I have seen this failure on the CI on ubuntu:rolling and on Ubuntu's autopkgtest on Ubuntu devel. I can reproduce it locally with 10–50% failure rate.

Here is one failing autopkgtest log: https://autopkgtest.ubuntu.com/results/autopkgtest-stonking/stonking/amd64/d/dracut/20260908_205705_e00a1@/log.gz

Distribution used
Ubuntu 26.04 (ubuntu:rolling) and 26.10 (ubuntu:devel)

Dracut version
main

Init system
systemd

To Reproduce
Run:

test/test.sh ubuntu:rolling 46

Expected behavior
The test should always succeed.

Additional context
Bug-Ubuntu: https://launchpad.net/bugs/2167284