feat: add py-pip to env

Let's put py-pip into the ESD as use cases often rely on pip install … and we can't rely on the system-installed version of pip. (I.e. it's plain luck that those use cases worked before.)

(FYI @kindler)

packages/wf-custom-python/package.py

+# Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class WfCustomPython(BundlePackage):
+    """Meta-package to represent a use case where users install additional
+    (on top of the ESD) Python packages."""
+
+    version("0.1")
+    
+    depends_on("py-pip")
+    depends_on("py-virtualenv")