ukify: when decompressing kernel before signing, call verify on decompressed file

ukify: when decompressing kernel before signing, call verify on decompressed file

Otherwise it will fail as it's an archive, not a PE file:

Invalid DOS header magic
Can't open image /boot/vmlinuz.old
/boot/vmlinuz.old is compressed and cannot be loaded by UEFI, decompressing
+ sbverify --list /boot/vmlinuz.old
=========================== short test summary info ============================
FAILED ../src/ukify/test/test_ukify.py::test_efi_signing_sbsign[3650] - subprocess.CalledProcessError: Command '['sbverify', '--list', PosixPath('/boot/vmlinuz.old')]' returned non-zero exit status 1.
FAILED ../src/ukify/test/test_ukify.py::test_efi_signing_sbsign[None] - subprocess.CalledProcessError: Command '['sbverify', '--list', PosixPath('/boot/vmlinuz.old')]' returned non-zero exit status 1.
FAILED ../src/ukify/test/test_ukify.py::test_inspect - subprocess.CalledProcessError: Command '['sbverify', '--list', PosixPath('/boot/vmlinuz.old')]' returned non-zero exit status 1.

Follow-up for 0dd03215f1e402092f6c6da213708045e445a9ed