Disable Buildkite commit verification for google-bazel-presubmit (#2890)

New images pick up **Buildkite agent v4** (the agent is installed
unpinned from the apt `stable` channel / GitHub `releases/latest`).
Agent v4 defaults `git-commit-verification` to `strict`, which breaks
every `google-bazel-presubmit` job that lands on a new VM, e.g.
https://buildkite.com/bazel/google-bazel-presubmit/builds/110111:

```
$ git merge-base --is-ancestor 5f1bfb3f... 864445577c...
🚨 Error: commit verification failed: commit "5f1bfb3f..." is not on branch "master"
```

Those builds are created with `branch=master` but check out an
unsubmitted Gerrit change, which is never an ancestor of master. Agents
still on 3.137.0 pass, so failures are intermittent while old and new
VMs are mixed.

### Change
Set `BUILDKITE_GIT_COMMIT_VERIFICATION=off` for `google-bazel-presubmit`
only, in the agent `environment` hook on Linux
(`startup-docker-pdssd.sh`) and Windows (`startup-windows-pdssd.ps1`).