Reduce the number of env vars exposed to subprocess (#1011)

The following env vars are no longer exposed to subprocesses run by the
buildpack (such as the `bin/pre_compile` and `bin/post_compile` hooks):

* `BPLOG_PREFIX`
* `CACHED_PYTHON_STACK`
* `DEFAULT_PYTHON_STACK`
* `DEFAULT_PYTHON_VERSION`
* `LATEST_27`
* `LATEST_34`
* `LATEST_35`
* `LATEST_36`
* `LATEST_37`
* `LATEST_38`
* `PIP_UPDATE`
* `PY27`
* `PY34`
* `PY35`
* `PY36`
* `PY37`
* `PYPY_27`
* `PYPY_36`
* `RECOMMENDED_PYTHON_VERSION`
* `WARNINGS_LOG`

There were previously no tests at all for the pre/post-compile hooks,
so I've added some now.

Fixes #1010.

test/fixtures/hooks/bin/post_compile

@@ -0,0 +1,4 @@
+set -euo pipefail
+
+echo "post_compile ran!"
+echo "post_compile env: $(printenv | cut -d '=' -f 1 | sort | xargs)."

test/fixtures/hooks/bin/pre_compile

@@ -0,0 +1,4 @@
+set -euo pipefail
+
+echo "pre_compile ran!"
+echo "pre_compile env: $(printenv | cut -d '=' -f 1 | sort | xargs)."