Missing debug output for commands run by subprocess.check_output

running diffoscope --debug doesn't show information about commands run from subprocess.check_output calls in diffoscope code, see for example in: try.diffoscope.org#13 (comment 170857)

The code that call the unsquashfs is at: https://salsa.debian.org/reproducible-builds/diffoscope/-/blob/5b8e075388b343a4fdc83548d51cdf9225a2c8b9/diffoscope/comparators/squashfs.py#L261-276 but there no mention of it being executed in debug output.

fix: for each subprocess.check_output call in code, add debug message with the executed command.