Variables are not substituted in the LDAP hook description

The issue was found by @piotrek during 1.18 sanity checks: #1460 (comment 479573)

LDAP hook deb package is showing strange description (variables not evaluated):

$ dpkg -I ~/Downloads/isc-stork-server-hook-ldap_1.18.0.240731155706_amd64.deb 
 new Debian package, version 2.0.
 size 5634688 bytes: control archive=428 bytes.
       1 bytes,     0 lines      conffiles            
     300 bytes,     9 lines      control              
      84 bytes,     1 lines      md5sums              
 Package: isc-stork-server-hook-ldap
 Version: 1.18.0.240731155706
 Section: admin
 Priority: required
 Architecture: amd64
 Maintainer: Stork team <stork-users@lists.isc.org>
 Installed-Size: 11221
 Homepage: https://stork.isc.org
 Description: ISC Stork ${STORK_NFPM_HOOK_KIND} ${STORK_NFPM_HOOK_NAME} hook