Print test-suite.log correctly in tarball system test job
Printing test-suite.log on system test failure does not work for system test run from tarball because the "after_script" step does not honour directory change from the "before_script" step and fails with:
Running after script...
$ cat bin/tests/system/test-suite.log
cat: bin/tests/system/test-suite.log: No such file or directory
Validation job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/1090455.