upgrade env in two steps

upgrade Python on its own because `conda env update` can get sad when upgrading Python along with a bunch of other transactions (specifically pip-install)

repo2docker/buildpacks/conda/install-miniconda.bash

@@ -20,31 +20,43 @@ if ! echo "${MD5SUM}  ${INSTALLER_PATH}" | md5sum  --quiet -c -; then
 fi
 
 bash ${INSTALLER_PATH} -b -p ${CONDA_DIR}
+export PATH="${CONDA_DIR}/bin:$PATH"
 
 # Allow easy direct installs from conda forge
-${CONDA_DIR}/bin/conda config --system --add channels conda-forge
+conda config --system --add channels conda-forge
 
 # Do not attempt to auto update conda or dependencies
-${CONDA_DIR}/bin/conda config --system --set auto_update_conda false
+conda config --system --set auto_update_conda false
+conda config --system --set show_channel_urls true
+
+# switch Python in its own step
+# since switching Python during an env update can
+# prevent pip installation.
+# we wouldn't have this issue if we did `conda env create`
+# instead of `conda env update` in these cases
+conda install -y $(cat /tmp/environment.yml | grep -o '\spython=.*')
+
 # bug in conda 4.3.>15 prevents --set update_dependencies
 echo 'update_dependencies: false' >> ${CONDA_DIR}/.condarc
-${CONDA_DIR}/bin/conda config --system --set show_channel_urls true
 
-# disable pip cache for conda-triggered pip installs
-export PIP_CACHE_DIR=0
+echo "installing root env:"
+cat /tmp/environment.yml
+conda env update -n root -f /tmp/environment.yml
 
-${CONDA_DIR}/bin/conda env update -n root -f /tmp/environment.yml
 # enable nteract-on-jupyter, which was installed with pip
 jupyter serverextension enable nteract_on_jupyter --sys-prefix
 
 if [[ -f /tmp/kernel-environment.yml ]]; then
     # install kernel env and register kernelspec
-    ${CONDA_DIR}/bin/conda env create -n kernel -f /tmp/kernel-environment.yml
+    echo "installing kernel env:"
+    cat /tmp/kernel-environment.yml
+
+    conda env create -n kernel -f /tmp/kernel-environment.yml
     ${CONDA_DIR}/envs/kernel/bin/ipython kernel install --prefix "${CONDA_DIR}"
 fi
 
 # Clean things out!
-${CONDA_DIR}/bin/conda clean -tipsy
+conda clean -tipsy
 
 # Remove the big installer so we don't increase docker image size too much
 rm ${INSTALLER_PATH}