Prevent CI (and runtime) apksigner test failures due to lack of binfmt_misc on...

Prevent CI (and runtime) apksigner test failures due to lack of binfmt_misc on Salsa CI and elsewhere.

In Debian, the `apksigner` binary is a symbolic link to the .jar file itself,
requiring binfmt_misc support to execute directly. We therefore resolve its
location and pass that to `java -jar` instead