[CI] Jobs being killed during execution in new premerge

[boomanaiden154]

We are seeing some workflows being killed prematurely in the new premerge system, typically in the build and test stage:

1. https://github.com/llvm/llvm-project/actions/runs/12719721823
2. https://github.com/llvm/llvm-project/actions/runs/12718965069
3. https://github.com/llvm/llvm-project/actions/runs/12718483685
4. https://github.com/llvm/llvm-project/actions/runs/12717546963
5. https://github.com/llvm/llvm-project/actions/runs/12712443995

Our original investigation pointed us to autoscaling, with logs showing that the nodes that a pod would be scheduled on were scheduled to downscale while the job was executing. This led to patches like 69508a6 which still did not fix the problem. It was still occurring, eg in https://github.com/llvm/llvm-project/actions/runs/12746855571.

Spending a lot more time looking at the logs, I observed that a konnectivity-agent pod would get killed about half a second before the job get prematurely killed/interrupted, with good consistency. They seemed to be going down on other nodes as they autoscaled down. We disabled autoscaling for a couple days to test this hypothesis and observed no failures. This being the issue could make sense given konnectivity handles communications between pods and the control plane, which we have to go through when running exec on a k8s pod.

The plan is to disable the kubernetes execution mode. This takes away some flexibility, but should fix the reliability problem. We also want to open an upstream issue in konnectivity so we can hopefully get the flexibility back.

Crossref for the internal bug: b/389220221.