Increase ram usage estimate in multiproduct_kati

multiproduct_kati assumed each build would take ~30GB of RAM,
but it seems it's closer to 40GB now.

The current build_test builds use 7 jobs, so this would bring
us down to 5, which should still keep us under the 6 hour
build time limit as currently these tests are at ~3 hours.

Bug: 262629589
Test: Presubmits
Change-Id: I51af364cc602a3bf7ba85fd8f92c4dd57585abbd