Drop parallel build from stress tests

Drop parallel build from stress tests

BUILD_PARALLEL_JOBS environmental variable is set to 6, which does not
align well with 4 and 8 CPU core systems dedicated to CI "stress" tests.
When multiple parallel jobs run on the host, they compete for resources
with an undesirable result: 6 compiler processes of one job may starve
named, resulting in lower-than-expected throughput and minutes-long
query response latency spikes.

Better drop the build parallelism of BIND-under-test. About 1-2 minutes
are added to the 60-65 minutes long job duration.