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.

bin/compile

@@ -16,7 +16,9 @@
 set -eo pipefail
 
 # Boostrap the Buildpack Standard Library.
-export BPLOG_PREFIX="buildpack.python"
+# Disable unused env var warning since shellcheck doesn't know about the stdlib.
+# shellcheck disable=2034
+BPLOG_PREFIX="buildpack.python"
 export BUILDPACK_LOG_FILE=${BUILDPACK_LOG_FILE:-/dev/null}
 
 [ "$BUILDPACK_XTRACE" ] && set -o xtrace
@@ -84,16 +86,12 @@ if [[ -f "$BUILD_DIR/Pipfile" ]]; then
     PIP_UPDATE="9.0.2"
 fi
 
-export DEFAULT_PYTHON_STACK PIP_UPDATE
-export PY37 PY36 PY35 PY27 PY34
-
 # Common Problem Warnings:
 # This section creates a temporary file in which to stick the output of `pip install`.
 # The `warnings` subscript then greps through this for common problems and guides
 # the user towards resolution of known issues.
 WARNINGS_LOG=$(mktemp)
-export WARNINGS_LOG
-export RECOMMENDED_PYTHON_VERSION=$DEFAULT_PYTHON_VERSION
+RECOMMENDED_PYTHON_VERSION=$DEFAULT_PYTHON_VERSION
 
 # The buildpack ships with a few executable tools (e.g. pip-grep, etc).
 # This installs them into the path, so we can execute them directly.
@@ -217,8 +215,6 @@ else
   CACHED_PYTHON_STACK=$STACK
 fi
 
-export CACHED_PYTHON_STACK
-
 # Pipenv Python version support.
 # Detect the version of Python requested from a Pipfile (e.g. python_version or python_full_version).
 # Convert it to a runtime.txt file.

bin/default_pythons

@@ -1,5 +1,10 @@
 #!/usr/bin/env bash
 
+# Disable unused env var warning, since shellcheck doesn't take into account
+# that this file is sourced. We don't want to use export since it exposes
+# the env vars to subprocesses.
+# shellcheck disable=2034
+
 DEFAULT_PYTHON_VERSION="python-3.6.11"
 LATEST_38="python-3.8.5"
 LATEST_37="python-3.7.8"
@@ -9,13 +14,3 @@ LATEST_34="python-3.4.10"
 LATEST_27="python-2.7.18"
 PYPY_36="pypy3.6-7.3.1"
 PYPY_27="pypy2.7-7.3.1"
-
-export DEFAULT_PYTHON_VERSION \
-    LATEST_38 \
-    LATEST_37 \
-    LATEST_36 \
-    LATEST_35 \
-    LATEST_34 \
-    LATEST_27 \
-    PYPY_36 \
-    PYPY_27