TEST-72-SYSUPDATE: make randomly generated image file not have compression header

TEST-72-SYSUPDATE: make randomly generated image file not have compression header

Otherwise, the generated image may be wrongly detected as compressed,
and importing the image may fail:
```
[   35.194578] TEST-72-SYSUPDATE.sh[411]: + dd if=/dev/urandom of=/var/tmp/test-72-N7uTeO/source/part1-v5.raw bs=4096 count=2048
[   35.236342] TEST-72-SYSUPDATE.sh[1075]: 2048+0 records in
[   35.236342] TEST-72-SYSUPDATE.sh[1075]: 2048+0 records out
[   35.236342] TEST-72-SYSUPDATE.sh[1075]: 8388608 bytes (8.4 MB, 8.0 MiB) copied, 0.0408601 s, 205 MB/s
(snip)
[   35.948634] TEST-72-SYSUPDATE.sh[1085]: \ Acquiring /var/tmp/test-72-N7uTeO/source/part1-v5.raw → /proc/self/fd/3p2...
[   35.952878] TEST-72-SYSUPDATE.sh[1085]: Successfully forked off '(sd-import-raw)' as PID 1089.
[   35.958952] TEST-72-SYSUPDATE.sh[1089]: Importing '/var/tmp/test-72-N7uTeO/source/part1-v5.raw', saving at offset 9437184 in '/dev/loop0'.
[   35.959575] TEST-72-SYSUPDATE.sh[1089]: Failed to decode and write: Input/output error
[   35.959575] TEST-72-SYSUPDATE.sh[1089]: Exiting.
```

Fixes #38524.

(cherry picked from commit 535539222d0636fade66236d4d1603f49a6c7a13)