Only set PIP_PYTHON at build time (#307)

Since it's not needed at run time after #264.

GUS-W-17444427.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- The `PIP_PYTHON` env var is now only set at build time. ([#307](https://github.com/heroku/buildpacks-python/pull/307))
+
 ### Removed
 
 - Stopped setting the `LANG` env var. ([#306](https://github.com/heroku/buildpacks-python/pull/306))

src/layers/pip_dependencies.rs

@@ -54,11 +54,11 @@ pub(crate) fn install_dependencies(
     .map_err(PipDependenciesLayerError::CreateVenvCommand)?;
 
     let mut layer_env = LayerEnv::new()
-        // Since pip is installed in a different layer (outside of this venv), we have to explicitly
-        // tell it to perform operations against this venv instead of the global Python install.
+        // pip is installed in a separate build-only layer, we have to explicitly tell it to
+        // perform operations against this venv instead of the global Python install.
         // https://pip.pypa.io/en/stable/cli/pip/#cmdoption-python
         .chainable_insert(
-            Scope::All,
+            Scope::Build,
             ModificationBehavior::Override,
             "PIP_PYTHON",
             &layer_path,

tests/pip_test.rs

@@ -85,7 +85,6 @@ fn pip_basic_install_and_cache_reuse() {
             formatdoc! {"
                 LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib
                 PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
-                PIP_PYTHON=/layers/heroku_python/venv
                 PYTHONHOME=/layers/heroku_python/python
                 PYTHONUNBUFFERED=1
                 VIRTUAL_ENV=/layers/heroku_python/venv