Add pre-exit hook to fix permissions in BuildKite

[frouioui]

This PR fixes a long-standing issue we have with our CI. Since we change the permission of certain files that are under the cloned directory, we may fail to clone/update the repo on the runner in a subsequent run. This is a pretty annoying issue as it fails our workflows very often.

Examples of the logs:

  [2024-10-15T05:52:34Z] $ git clone -v -- https://github.com/planetscale/vitess-operator.git .
  --
  | [2024-10-15T05:52:34Z] fatal: destination path '.' already exists and is not an empty directory.
  | [2024-10-15T05:52:34Z] ⚠️ Warning: Checkout failed! cloning git repository: exit status 128 (Attempt 1/3 Retrying in 2s)
  | [2024-10-15T05:52:34Z] # Removing /var/lib/buildkite-agent/builds/buildkite-public-i-06a8b5ff3b7b45bbd-1/planetscale/vitess-operator
  | [2024-10-15T05:52:34Z] 🚨 Error: Failed to remove "/var/lib/buildkite-agent/builds/buildkite-public-i-06a8b5ff3b7b45bbd-1/planetscale/vitess-operator" (unlinkat /var/lib/buildkite-agent/builds/buildkite-public-i-06a8b5ff3b7b45bbd-1/planetscale/vitess-operator/vtdataroot/backup/example/commerce/-: permission denied)
  | [2024-10-15T05:52:34Z] # Waiting 10 seconds
  | [2024-10-15T05:52:44Z] # Removing /var/lib/buildkite-agent/builds/buildkite-public-i-06a8b5ff3b7b45bbd-1/planetscale/vitess-operator
  | [2024-10-15T05:52:44Z] 🚨 Error: Failed to remove "/var/lib/buildkite-agent/builds/buildkite-public-i-06a8b5ff3b7b45bbd-1/planetscale/vitess-operator" (unlinkat /var/lib/buildkite-agent/builds/buildkite-public-i-06a8b5ff3b7b45bbd-1/planetscale/vitess-operator/vtdataroot/backup/example/commerce/-: permission denied)
  | [2024-10-15T05:52:44Z] # Waiting 10 seconds

[frouioui]

Had to push one more commit to fix some docs that was missing from a previous PR. Context: the HPA PR was merged without being up-to-date with main, main now has a workflow that checks docs are generated correctly.