Upgrade rust/crates/structopt to 0.3.20

Test: make
Change-Id: I50a53b96c652aea7c84a572a87578cf3828c00e2
diff --git a/METADATA b/METADATA
index b6bc6ff..b4901c8 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/structopt/structopt-0.3.18.crate"
+    value: "https://static.crates.io/crates/structopt/structopt-0.3.20.crate"
   }
-  version: "0.3.18"
+  version: "0.3.20"
   license_type: NOTICE
   last_upgrade_date {
     year: 2020
-    month: 9
-    day: 23
+    month: 10
+    day: 26
   }
 }