Disable pip's version check + cache when installing pip/setuptools/wh…

…eel (#1007)

Since the version check is redundant given we control/choose the version.

The pip cache is redundant since we instead cache site-packages. The pip
cache also ends up in `/app` so isn't included in the build cache anyway.

CHANGELOG.md

@@ -6,6 +6,7 @@
 - Install an explicit version of wheel rather than the latest release at the time (#1007).
 - Output the installed version of pip, setuptools and wheel in the build log (#1007).
 - Install pip using itself rather than `get-pip.py` (#1007).
+- Disable pip's version check + cache when installing pip/setuptools/wheel (#1007).
 - Install setuptools from PyPI rather than a vendored copy (#1007).
 - Reduce the number of environment variables exposed to `bin/{pre,post}_compile` and other subprocesses (#1011)
 

bin/steps/python

@@ -174,7 +174,8 @@ fi
 
 puts-step "Installing pip ${PIP_VERSION}, setuptools ${SETUPTOOLS_VERSION} and wheel ${WHEEL_VERSION}"
 
-/app/.heroku/python/bin/python "${PIP_WHEEL}/pip" install "${PIP_TO_INSTALL}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}" &> /dev/null
+/app/.heroku/python/bin/python "${PIP_WHEEL}/pip" install --disable-pip-version-check --no-cache \
+  "${PIP_TO_INSTALL}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}" &> /dev/null
 
 set -e
 hash -r