only install pipenv in preassemble

save preassembly of pipenv for a later patch

repo2docker/buildpacks/pipfile/__init__.py

@@ -75,6 +75,15 @@ class PipfileBuildPack(CondaBuildPack):
         return files
 
     def get_preassemble_scripts(self):
+        """scripts to run prior to staging the repo contents"""
+        scripts = super().get_preassemble_scripts()
+        # install pipenv to install dependencies within Pipfile.lock or Pipfile
+        scripts.append(
+            ("${NB_USER}", "${KERNEL_PYTHON_PREFIX}/bin/pip install pipenv==2018.11.26")
+        )
+        return scripts
+
+    def get_assemble_scripts(self):
         """Return series of build-steps specific to this repository.
         """
         # If we have either Pipfile.lock, Pipfile, or runtime.txt declare the
@@ -113,11 +122,6 @@ class PipfileBuildPack(CondaBuildPack):
         #     my_package_example = {path=".", editable=true}
         working_directory = self.binder_dir or "."
 
-        # install pipenv to install dependencies within Pipfile.lock or Pipfile
-        assemble_scripts.append(
-            ("${NB_USER}", "${KERNEL_PYTHON_PREFIX}/bin/pip install pipenv==2018.11.26")
-        )
-
         # NOTES:
         # - Without prioritizing the PATH to KERNEL_PYTHON_PREFIX over
         #   NB_SERVER_PYTHON_PREFIX, 'pipenv' draws the wrong conclusion about