Fix flaky file detection

magic sometimes misdetect the file type[1] as seen with the haskell-gi-xlib-2.0.11-15-x86_64.pkg.tar.zst package in our case (detected as "DOS/MBR boot sector").

Workaround the issue by letting python-magic "keep going" (i.e. don't stop on first match) and check if one of the matches is the expected type. file-magic unfortunately doesn't have this option.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876316

Fix #146 (closed)