patch 8.0.1640: Test_cwd() is flaky

Problem:    Test_cwd() is flaky.
Solution:   Add to list of flaky tests.
diff --git a/src/version.c b/src/version.c
index 4b43e49..d4631ce 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1640,
+/**/
     1639,
 /**/
     1638,