fix: point controller at milo via KUBECONFIG env var

[ecv]

Summary

Set `KUBECONFIG` on the manager container in the `milo-integration` component so the controller reads the mounted discovery kubeconfig instead of falling back to the pod's in-cluster ServiceAccount. client-go honors `KUBECONFIG` natively — no flag, config type, or ConfigMap needed.

The component already mounts `inventory-discovery-kubeconfig` at `/etc/kubernetes/project-discovery/auth`; this just tells client-go to use it.

Fixes #9.

Test plan

• `kustomize build` of a synthetic overlay including `components/milo-integration` renders the manager container with `KUBECONFIG=/etc/kubernetes/project-discovery/auth/discovery-kubeconfig.yaml`
• Post-merge: confirm inventory-controller-manager reaches Ready against the Milo apiserver in staging

[ecv]

This can't be the right way to do this @scotwells . Unless…

[scotwells]

@ecv that's one way to do it. Could also just set the KUBECONFIG variable to the kubeconfig path to use for connecting to milo's control plane.

[ecv]

Unless I misunderstand @scotwells this repository lacks the wiring for kubeconfigPath, and this PR (now) creates that wiring w/ a sane default that shouldn't need patching in the infra repo.

It's entirely probable that I misunderstand. Clarifications welcomed.

[scotwells]

@ecv by default the k8s libraries will load the config from the KUBECONFIG env variable.

https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/#the-kubeconfig-environment-variable

The approach here works too, it's just a more explicit configuration option than the env variable.

[ecv]

Oh. You mean. Like that. @scotwells