fix(toolchain): Also set Make variables for local toolchains (#3641)

Currently, the toolchain only sets the make variables if it's an
in-build toolchain.
This breaks for platform toolchains.

To fix, check for platform toolchains and set the make vars
appropriately.

This helps fix the BCR rules for `@glib`, which recently added a
requirement that
the Make variable `$(PYTHON3)` must always be defined by the current
Python toolchain.

Co-authored-by: Richard Levasseur <richardlev@gmail.com>

Author: jwnimmer-tri

CHANGELOG.md

@@ -63,8 +63,10 @@ END_UNRELEASED_TEMPLATE
 
 {#v0-0-0-fixed}
 ### Fixed
+* (toolchain) Also set Make variables for local toolchains.
 * (zipapp) Resolve issue passing through compression settings in
-  `py_zippapp_binary` targets. ([#3646](https://github.com/bazel-contrib/rules_python/issues/3646))
+  `py_zippapp_binary` targets
+  ([#3646](https://github.com/bazel-contrib/rules_python/issues/3646)).
 
 {#v0-0-0-added}
 ### Added

python/current_py_toolchain.bzl

@@ -28,12 +28,18 @@ def _current_py_toolchain_impl(ctx):
         transitive.append(toolchain.py3_runtime.files)
         vars["PYTHON3"] = toolchain.py3_runtime.interpreter.path
         vars["PYTHON3_ROOTPATH"] = toolchain.py3_runtime.interpreter.short_path
+    elif toolchain.py3_runtime and toolchain.py3_runtime.interpreter_path:
+        vars["PYTHON3"] = toolchain.py3_runtime.interpreter_path
+        vars["PYTHON3_ROOTPATH"] = toolchain.py3_runtime.interpreter_path
 
     if toolchain.py2_runtime and toolchain.py2_runtime.interpreter:
         direct.append(toolchain.py2_runtime.interpreter)
         transitive.append(toolchain.py2_runtime.files)
         vars["PYTHON2"] = toolchain.py2_runtime.interpreter.path
         vars["PYTHON2_ROOTPATH"] = toolchain.py2_runtime.interpreter.short_path
+    elif toolchain.py2_runtime and toolchain.py2_runtime.interpreter_path:
+        vars["PYTHON2"] = toolchain.py2_runtime.interpreter_path
+        vars["PYTHON2_ROOTPATH"] = toolchain.py2_runtime.interpreter_path
 
     files = depset(direct, transitive = transitive)
     return [