No need to install prereqs twice on circleci.com

pavelib/prereqs.py

@@ -11,6 +11,10 @@ from .utils.envs import Env
 
 PREREQS_MD5_DIR = os.getenv('PREREQ_CACHE_DIR', Env.REPO_ROOT / '.prereqs_cache')
 NPM_REGISTRY = "http://registry.npmjs.org/"
+
+# If you make any changes to this list you also need to make
+# a corresponding change to circle.yml, which is how the python
+# prerequisites are installed for builds on circleci.com
 PYTHON_REQ_FILES = [
     'requirements/edx/pre.txt',
     'requirements/edx/github.txt',

scripts/circle-ci-tests.sh

@@ -25,6 +25,10 @@ set -e
 # piped command, or a zero if they all succeed.
 set -o pipefail
 
+# There is no need to install the prereqs, as this was already
+# just done via the dependencies override section of circle.yml.
+export NO_PREREQ_INSTALL='true'
+
 EXIT=0
 
 case $CIRCLE_NODE_INDEX in