Let local users specify malioc path via environment (#40256)

Let local users specify malioc path via environment

tools/gn

diff --git a/tools/gn b/tools/gn
index 072a6d2..47664dd 100755
--- a/tools/gn
+++ b/tools/gn
@@ -559,6 +559,10 @@
 
   if args.malioc_path is not None:
     gn_args['impeller_malioc_path'] = args.malioc_path
+  else:
+    malioc_path = os.environ.get('MALIOC_PATH')
+    if malioc_path:
+      gn_args['impeller_malioc_path'] = malioc_path
 
   # ANGLE is exclusively used for:
   #  - Windows at runtime