test: call mksquashfs with -noappend to be safe on reruns

test: call mksquashfs with -noappend to be safe on reruns

If the test VM reboots and the test re-runs, creating the images
fails as they already exist:

[  218.227766] TEST-50-DISSECT.sh[889]: + mksquashfs testkit/ testkit.raw
[  218.238754] TEST-50-DISSECT.sh[2964]: FATAL ERROR: Could not read $HOME, use -recovery-path or -no-recovery options
[  218.239284] TEST-50-DISSECT.sh[2964]: Found a valid exportable SQUASHFS superblock on testkit.raw.
[  218.239554] TEST-50-DISSECT.sh[2964]: 	Compression used gzip
[  218.240176] TEST-50-DISSECT.sh[2964]: 	Inodes are compressed
[  218.240459] TEST-50-DISSECT.sh[2964]: 	Data is compressed
[  218.241072] TEST-50-DISSECT.sh[2964]: 	Fragments are compressed
[  218.241526] TEST-50-DISSECT.sh[2964]: 	Xattrs are compressed
[  218.241953] TEST-50-DISSECT.sh[2964]: 	Fragments are present in the filesystem
[  218.242411] TEST-50-DISSECT.sh[2964]: 	Always-use-fragments option is not specified
[  218.242843] TEST-50-DISSECT.sh[2964]: 	Duplicates are removed
[  218.243560] TEST-50-DISSECT.sh[2964]: 	Xattrs are stored
[  218.243889] TEST-50-DISSECT.sh[2964]: 	Filesystem size 0.38 Kbytes (0.00 Mbytes)
[  218.244563] TEST-50-DISSECT.sh[2964]: 	Block size 131072
[  218.245051] TEST-50-DISSECT.sh[2964]: 	Number of fragments 1
[  218.245512] TEST-50-DISSECT.sh[2964]: 	Number of inodes 6
[  218.245851] TEST-50-DISSECT.sh[2964]: 	Number of ids 1
[  218.246393] TEST-50-DISSECT.sh[2964]: Parallel mksquashfs: Using 2 processors
[  218.246820] TEST-50-DISSECT.sh[2964]: Scanning existing filesystem...
[  218.247286] TEST-50-DISSECT.sh[2964]: Read existing filesystem, 5 inodes scanned
[  218.252974] TEST-50-DISSECT.sh[2964]: Appending to existing 4.0 filesystem on testkit.raw, block size 131072
[  218.253593] TEST-50-DISSECT.sh[2964]: All -b, -noI, -noD, -noF, -noX, -noId, -no-duplicates, -no-fragments,
[  218.253848] TEST-50-DISSECT.sh[2964]: -always-use-fragments, -exportable and -comp options ignored
[  218.257196] TEST-50-DISSECT.sh[2964]: If appending is not wanted, please re-run with -noappend specified!

https://github.com/systemd/systemd/actions/runs/17674609143/job/50233691148?pr=38867