Fix path to suppr-lsan.txt file

The "suppr-lsan.txt" file needs to be referenced with GitLab-specific variable, otherwise AddressSanitizer won't find it outside the "isc-projects" project group.

This has been introduced in 8a4f098d.

Reported in MM, example of failure:

AddressSanitizer: failed to read suppressions file '/builds/isc-projects/bind9/suppr-lsan.txt'
../ckdnsrps.sh: line 62:  3783 Aborted                 (core dumped) $FEATURETEST --enable-dnsrps

Validation runs:

• isc-projects - https://gitlab.isc.org/isc-projects/bind9/-/jobs/2269187
• isc-private - https://gitlab.isc.org/isc-private/bind9/-/jobs/2269195