gh-113119 fix environment handling in subprocess.Popen when posix_spawn is used (#113120)

* Allow posix_spawn to inherit environment form parent environ variable.

With this change, posix_spawn call can behave similarly to execv with regards to environments when used in subprocess functions.

Lib/subprocess.py

@@ -1756,9 +1756,6 @@ class Popen:
                          c2pread, c2pwrite,
                          errread, errwrite):
             """Execute program using os.posix_spawn()."""
-            if env is None:
-                env = os.environ
-
             kwargs = {}
             if restore_signals:
                 # See _Py_RestoreSignals() in Python/pylifecycle.c