fix(pypi): build the environment on the fly (bazel-contrib#3720)

At some point when we started using pipstar, we silently started
building the python environment with system python interpreter. This is
fine because the environment in those code paths would be unused, but
this would break if python was not present on the machine.

This PR fixes this by creating the environment on the fly with a little
bit of code duplication.

Fixes bazel-contrib#3712

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>
(cherry picked from commit 19a2c04)

Author: aignas

CHANGELOG.md

@@ -127,6 +127,8 @@ Other changes:
   )
   ```
   Fixes [#3676](https://github.com/bazel-contrib/rules_python/issues/3676).
+* (pypi) Fixes wheel extraction on hosts without python installed,
+  Fixes [#3712](https://github.com/bazel-contrib/rules_python/issues/3712).
 
 {#v2-0-0-added}
 ### Added

python/private/pypi/whl_library.bzl

@@ -306,9 +306,6 @@ def _whl_library_impl(rctx):
     extra_pip_args = []
     extra_pip_args.extend(rctx.attr.extra_pip_args)
 
-    # Manually construct the PYTHONPATH since we cannot use the toolchain here
-    environment = _create_repository_execution_environment(rctx, python_interpreter, logger = logger)
-
     whl_path = None
     sdist_filename = None
     if rctx.attr.whl_file:
@@ -361,6 +358,14 @@ def _whl_library_impl(rctx):
     enable_pipstar = (rp_config.enable_pipstar or whl_path) and rctx.attr.config_load
     enable_pipstar_extract = enable_pipstar and rp_config.bazel_8_or_later
 
+    # When pipstar is enabled, Python isn't used, so there's no need
+    # to setup env vars to run Python, unless we need to build an sdist
+    if enable_pipstar_extract and whl_path:
+        environment = {}
+    else:
+        # Manually construct the PYTHONPATH since we cannot use the toolchain here
+        environment = _create_repository_execution_environment(rctx, python_interpreter, logger = logger)
+
     if not whl_path:
         if rctx.attr.urls:
             op_tmpl = "whl_library.BuildWheelFromSource({name}, {requirement})"