Automatic merge from submit-queue (batch tested with PRs 63703, 63628). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Use absolute path for KUBECTL default in local up cluster

This avoids having to set the KUBECTL env var if local-up-cluster.sh is run outside of the repo root.



**What this PR does / why we need it**:

Avoids having to set the KUBECTL env var if local-up-cluster.sh is run outside of the repo root.

**Release note**:

```release-note
NONE
```