Increase Linux docs_publish timeout (#120718)

This target recently caused the tree to close due to a timeout: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20docs_publish/8906/overview

This target consistently takes ~55 minutes, which is very close to its current timeout of 60 minutes. This increases the timeout to make timeouts less likely.

.ci.yaml

@@ -303,7 +303,7 @@ targets:
   - name: Linux docs_publish
     recipe: flutter/flutter
     presubmit: false
-    timeout: 60
+    timeout: 90
     properties:
       cores: "32"
       dependencies: >-