Bump linux bazel cache timeout to 30 minutes (#139491) (#139493)

We're seeing some timeouts recently in the 15 minute range.
Mac timeouts are already at 60 min.

(cherry picked from commit a2f3e3bd7c)

Co-authored-by: Jonathan Budzenski <jon@elastic.co>

.buildkite/pipelines/on_merge.yml

@@ -111,7 +111,7 @@ steps:
     label: 'Populate local dev bazel cache (Linux)'
     agents:
       queue: n2-4-spot
-    timeout_in_minutes: 15
+    timeout_in_minutes: 30
     retry:
       automatic:
         - exit_status: '-1'