bpo-40854: Allow overriding sys.platlibdir via PYTHONPLATLIBDIR env-var (GH-20605) (GH-20725)

(cherry picked from commit 8f023a2f664f902a3d0b7a6f64d63afc0d1c15ae)

Co-authored-by: Sandro Mani <manisandro@gmail.com>
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index e3fe143..3e4115f 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -2922,13 +2922,7 @@
     SET_SYS_FROM_WSTR("base_prefix", config->base_prefix);
     SET_SYS_FROM_WSTR("exec_prefix", config->exec_prefix);
     SET_SYS_FROM_WSTR("base_exec_prefix", config->base_exec_prefix);
-    {
-        PyObject *str = PyUnicode_FromString(PLATLIBDIR);
-        if (str == NULL) {
-            return -1;
-        }
-        SET_SYS_FROM_STRING("platlibdir", str);
-    }
+    SET_SYS_FROM_WSTR("platlibdir", config->platlibdir);
 
     if (config->pycache_prefix != NULL) {
         SET_SYS_FROM_WSTR("pycache_prefix", config->pycache_prefix);