py-scikit-learn: do parallel builds of C extension (#20733)

Enabled parallel builds of the C-extension upto the parallelism allowed
by spack.

https://scikit-learn.org/stable/developers/advanced_installation.html#parallel-builds

var/spack/repos/builtin/packages/py-scikit-learn/package.py

@@ -62,6 +62,9 @@ class PyScikitLearn(PythonPackage):
     conflicts('~openmp', when='@:999', msg='Only master supports ~openmp')
 
     def setup_build_environment(self, env):
+        # enable parallel builds of the sklearn backend
+        env.append_flags("SKLEARN_BUILD_PARALLEL", str(make_jobs))
+
         # https://scikit-learn.org/stable/developers/advanced_installation.html#building-from-source
         if self.spec.satisfies('~openmp'):
             env.set('SKLEARN_NO_OPENMP', 'True')