CPack/Deb: Recognize shared libraries marked executable on GNU/Hurd

When running tests one test fails for GNU/Hurd since the generated file
`libtest_lib.so.0.8` is executable.  Currently it is unknown why it is
created executable.  However, some library files are being created
executable, e.g. `/lib/x86_64-linux-gnu/libc-2.27.so` is. So `file(1)`
is doing the right thing, to say that this file is ELF 64-bit LSB pie
executable instead of ELF 64-bit LSB shared object.

Fixes: #18376