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/Programs/_testembed.c b/Programs/_testembed.c
index 864a6e7..d89f6be 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -548,6 +548,13 @@
     /* FIXME: test home */
     /* FIXME: test path config: module_search_path .. dll_path */
 
+    putenv("PYTHONPLATLIBDIR=env_platlibdir");
+    status = PyConfig_SetBytesString(&config, &config.platlibdir, "my_platlibdir");
+    if (PyStatus_Exception(status)) {
+        PyConfig_Clear(&config);
+        Py_ExitStatusException(status);
+    }
+
     putenv("PYTHONVERBOSE=0");
     Py_VerboseFlag = 0;
     config.verbose = 1;
@@ -668,6 +675,7 @@
     putenv("PYTHONFAULTHANDLER=1");
     putenv("PYTHONIOENCODING=iso8859-1:replace");
     putenv("PYTHONOLDPARSER=1");
+    putenv("PYTHONPLATLIBDIR=env_platlibdir");
 }