libdpkg: Set umask to 0 when creating symlinks in test suite to normalize results

POSIX specifies that symlinks have undefined permissions in their mode,
so their setting is system dependent. Linux does not honor the umask
for symlinks, other systems like Hurd or kFreeBSD do, which means we
get different results due to this.

Regression introduced in commit f71e02c8.

Closes: #764171