Allow using -j to control the parallelism of concretization (#37608)

fixes #29464

This PR allows to use
```
$ spack concretize -j X
```
to set a cap on the parallelism of concretization from the command line

lib/spack/spack/cmd/concretize.py

@@ -29,6 +29,7 @@ def setup_parser(subparser):
     )
 
     spack.cmd.common.arguments.add_concretizer_args(subparser)
+    spack.cmd.common.arguments.add_common_arguments(subparser, ["jobs"])
 
 
 def concretize(parser, args):

lib/spack/spack/environment/environment.py

@@ -1475,7 +1475,10 @@ def _concretize_separately(self, tests=False):
 
         # Solve the environment in parallel on Linux
         start = time.time()
-        max_processes = min(len(arguments), 16)  # Number of specs  # Cap on 16 cores
+        max_processes = min(
+            len(arguments),  # Number of specs
+            spack.config.get("config:build_jobs"),  # Cap on build jobs
+        )
 
         # TODO: revisit this print as soon as darwin is parallel too
         msg = "Starting concretization"