[9.16] 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.

(cherry picked from commit 3fd7e7c8)

Backport of MR !7669 (merged)