misc/vault-keyring-client.sh: add flock workaround

Otherwise running terraform under tf-stage2 will often fail with:

> ansible.errors.AnsibleError: Vault password client script
> ../misc/vault-keyring-client.sh did not find a secret for
> vault-id=default: b'gpg: decryption failed: No secret key\n'
2 jobs for master in 32 seconds (queued for 2 seconds)