Play nicely with gst-build uninstalled

As overrides_hack is being used as user sitecustomize script

testsuite/overrides_hack.py

 import os
 import gi.overrides
 
+FILE = os.path.realpath(__file__)
 if not gi.overrides.__path__[0].endswith("gst-python/gi/overrides"):
     local_overrides = None
     # our overrides don't take precedence, let's fix it
@@ -11,6 +12,13 @@ if not gi.overrides.__path__[0].endswith("gst-python/gi/overrides"):
     if local_overrides:
         gi.overrides.__path__.remove(local_overrides)
     else:
-        local_overrides = os.path.abspath(os.path.join(__file__, "../", "../", "gi", "overrides"))
+        local_overrides = os.path.abspath(os.path.join(FILE, "../", "../", "gi", "overrides"))
 
     gi.overrides.__path__.insert(0, local_overrides)
+
+# Execute previously set sitecustomize.py script if it existed
+if os.environ.get("GST_ENV"):
+    old_sitecustomize = os.path.join(os.path.dirname(__file__),
+                                    "old.sitecustomize.gstuninstalled.py")
+    if os.path.exists(old_sitecustomize):
+        exec(compile(open(old_sitecustomize).read(), old_sitecustomize, 'exec'))