Set SYSTEMTESTTOP in eddsa ns3 sign script
eddsa/ns3/sign.sh script failed to set SYSTEMTESTTOP environmental
variable in 4538d8dd. Failing to set the
variable in scripts deeper than in root of system test fails to
correctly set TOP variable in conf.sh.win32 and thus fails to source
conf.sh.common.
Job with fix https://gitlab.isc.org/isc-projects/bind9/-/jobs/1497783#L4177.
/cc @matthijs