ci: run the OpenBSD --use-tcp test step at -j2

ci: run the OpenBSD --use-tcp test step at -j2

The OpenBSD job runs inside a nested VM. At -j8 the --use-tcp run starts
many concurrent loopback daemons, and under that resource pressure the
daemon connection handshake occasionally loses a timing race and one test
hangs to the 300s runner timeout. It is an environment artifact, not an
rsync defect: the daemon handshake writes-then-reads with unbuffered early
I/O (no flush/mutual-wait deadlock), the indefinite wait is the documented
no-timeout daemon behaviour, and it does not reproduce off OpenBSD even with
the full suite pinned to a single CPU at -j8.

Drop just this job's --use-tcp parallelism to -j2 so the nested VM stops
over-subscribing; the pipe `make check` and every other platform are
unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>