Prevent OpenBLAS from spawning worker threads. TNL-6456

The RLIMIT_NPROC limit is per-user so simultaneous student submissions
pile up and overflow the limit.  Setting this environment variable will
prevent spawning any threads (the 1 means only have one threads, the
already existing main thread).

common/lib/capa/capa/safe_exec/safe_exec.py

@@ -14,6 +14,9 @@ import hashlib
 CODE_PROLOG = """\
 from __future__ import division
 
+import os
+os.environ["OPENBLAS_NUM_THREADS"] = "1"    # See TNL-6456
+
 import random as random_module
 import sys
 random = random_module.Random(%r)