Fix Linux VM image creation and handle gcloud SQLite concurrency collisions (#2769)

### Summary of Changes

1. **`buildkite/create_images.py`**:
- Use machine type `c4a-standard-8` for the `bk-testing-docker-arm64`
image builder VM. Local SSDs (`-lssd`) are ephemeral, not captured in
custom GCE images, and caused capacity stockouts
(`ZONE_RESOURCE_POOL_EXHAUSTED_WITH_DETAILS`) in `us-central1`.

2. **`buildkite/gcloud.py`**:
- Add automatic retry for transient `gcloud crashed (OperationalError)`
/ SQLite schema migration collisions when multiple threads invoke
`gcloud` concurrently.
- Example failure:
https://buildkite.com/bazel-trusted/create-linux-vm-image/builds/43#019ff552-6791-4b7a-95c4-2c7c882bcfd5