crostini: Extend use-buster expiry

It's enabled by default, but we still support Stretch so toggling the
flag false is useful for testing.

Bug: None
Test: None
Change-Id: I0dc8203835618166fb4df339c649fa8dff33992a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426211
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Fergus Dall <sidereal@google.com>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#810156}

chrome/browser/flag-metadata.json

@@ -699,7 +699,7 @@
   {
     "name": "crostini-use-buster-image",
     "owners": [ "davidmunro@google.com", "benwells" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 90
   },
   {
     "name": "crostini-use-dlc",