[CI] Increase artifact build timeout (elastic#183482)

## Summary
Increases step timeout for building the whole artifact collection by
15m.

With some recent additions ([chainguard
build](elastic#183200) adds ~7m) and the
new infra overhead, we've gone from ~50-52 minutes to ~57-60 minutes
(this one timed out exactly on the last bit:
https://buildkite.com/elastic/kibana-artifacts-snapshot/builds/4295#018f7b4c-3629-4c4f-8d80-85b2552a43c4)

.buildkite/pipelines/artifacts.yml

@@ -6,7 +6,7 @@ steps:
       imageProject: elastic-images-qa
       provider: gcp
       machineType: c2-standard-16
-    timeout_in_minutes: 60
+    timeout_in_minutes: 75
     retry:
       automatic:
         - exit_status: '*'