when running pip install tests, cache downloads from pypi

Since this runs frequently without much change, it makes sense to cache the
downloaded source packages as much as possible.  There are probably better
ways to do this, but this is really easy.

https://stackoverflow.com/questions/4806448/how-do-i-install-from-a-local-cache-with-pip

jenkins-build

@@ -25,6 +25,14 @@ if [ -z $ANDROID_HOME ]; then
     fi
 fi
 
+
+#------------------------------------------------------------------------------#
+# cache pypi downloads
+if [ -z $PIP_DOWNLOAD_CACHE ]; then
+    export PIP_DOWNLOAD_CACHE=$HOME/.pip_download_cache
+fi
+
+
 #------------------------------------------------------------------------------#
 # required Java 7 keytool/jarsigner for :file support