Explicitly specify where npm is

PATH doesn't seem to be set at this point to include
the conda environment we just installed?

repo2docker/buildpacks/conda/__init__.py

@@ -99,7 +99,7 @@ class CondaBuildPack(BaseImage):
             (
                 "${NB_USER}",
                 r"""
-                npm config --global set prefix ${NPM_DIR}
+                ${NB_PYTHON_PREFIX}/bin/npm config --global set prefix ${NPM_DIR}
                 """,
             ),
         ]