Set NVC_CONCURRENT_JOBS to value of num_threads argument

See VUnit#1036 and nickg/nvc@0758f79d7c6c

docs/news.d/1036.feature.rst

@@ -0,0 +1,4 @@
+[NVC] VUnit now sets the ``NVC_CONCURRENT_JOBS`` environment variable to
+the value of the ``--num-threads`` argument which allows ``nvc`` to
+scale its worker thread count based on the number of concurrently
+running simulations.

vunit/sim_if/nvc.py

@@ -57,6 +57,7 @@ def from_args(cls, args, output_path, **kwargs):
             output_path=output_path,
             prefix=prefix,
             gui=args.gui,
+            num_threads=args.num_threads,
         )
 
     @classmethod
@@ -70,6 +71,7 @@ def __init__(  # pylint: disable=too-many-arguments
         self,
         output_path,
         prefix,
+        num_threads,
         gui=False,
         gtkwave_args="",
     ):
@@ -91,6 +93,10 @@ def __init__(  # pylint: disable=too-many-arguments
         if self.use_color:
             environ["NVC_COLORS"] = "always"
 
+        # Allow NVC to scale its worker thread count based on the number
+        # of VUnit threads and the number of available CPUs.
+        environ["NVC_CONCURRENT_JOBS"] = str(num_threads)
+
     def has_valid_exit_code(self):  # pylint: disable=arguments-differ
         """
         Return if the simulation should fail with nonzero exit codes