Use gzip -d instead of gunzip

GNU gunzip [1] is a shell script that exec's `gzip -d`. Even if we call
/usr/bin/gunzip with the correct built-in path, the actual gzip call
will use whichever gzip it finds first, making our patch pointless.

Fix this by explicitly calling gzip -d instead.

https://git.savannah.gnu.org/cgit/gzip.git/tree/gunzip.in



[Part of the fix for CVE-2022-4883]
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>