fix pam_path variable assignment during build

[Atomsoldat]

It seems that the path for the PAM config file for ly gets determined differently than the other paths. This does not normally cause any problems, because prefixing an empty string to config_directory still results in the correct directory. But it breaks when a non-default dest_directory is used.

Reproduce via:

zig build installsystemd -Ddest_directory='/tmp/debug-ly' && find /tmp/debug-ly

Note the duplicated path:

/tmp/debug-ly
...
/tmp/debug-ly/tmp
/tmp/debug-ly/tmp/debug-ly
/tmp/debug-ly/tmp/debug-ly/etc
/tmp/debug-ly/tmp/debug-ly/etc/pam.d
/tmp/debug-ly/tmp/debug-ly/etc/pam.d/ly

This MR should fix that.