signingscript: sign_authenticode_file can't check if MSIX is already signed

[agashlin]

Since mozilla-releng/winsign#23 , winsign supports signing MSIX files with makemsix. signingscript checks whether a file is already signed with winsign.osslsigncode.is_signed(), which is not effective with the Zip-based MSIX. winsign should probably expose a generic is_signed() function that can check PE/MSI via winsign.osslsigncode.is_signed() or MSIX via winsign.makemsix.is_signed() (which does not yet exist).

osslsigncode just can't make sense of the file, is_signed() always returns False, so I don't think this causes any issues.