fix: Fix whl_library in bazel vendor mode

- Added PYTHONHOME to whl_library execution enviornment. Without
it the python interpretter is getting confused where it's running
from when bazel --vendor_dir is used
- In _get_toolchain_unix_cflags, the real path for python is used
instead of the symlink.

Author: Omar Aldrroubi

python/private/pypi/whl_library.bzl

@@ -109,7 +109,11 @@ def _get_toolchain_unix_cflags(rctx, python_interpreter, logger = None):
     stdout = pypi_repo_utils.execute_checked_stdout(
         rctx,
         op = "GetPythonVersionForUnixCflags",
-        python = python_interpreter,
+        # python_interpreter by default points to a symlink, however when using bazel in vendor mode,
+        # and the vendored directory moves around, the execution of python fails, as it's getting confused
+        # where it's running from. More to the fact that we are executing it in isolated mode "-I", which
+        # results in PYTHONHOME being ignored. The solution is to run python from it's real directory.
+        python = python_interpreter.realpath,
         arguments = [
             # Run the interpreter in isolated mode, this options implies -E, -P and -s.
             # Ensures environment variables are ignored that are set in userspace, such as PYTHONPATH,
@@ -214,6 +218,7 @@ def _create_repository_execution_environment(rctx, python_interpreter, logger =
             rctx,
             entries = rctx.attr._python_path_entries,
         ),
+        "PYTHONHOME": str(python_interpreter.dirname),
     }
 
     # Gather any available CPPFLAGS values