This allows getting the Vault token from the `VAULT_TOKEN` env var or
from the file `$HOME/.vault-token`, as both of these are understood by
the Vault CLI and are a common place to put Vault tokens. This allows
avoiding hard-coding a Vault token into playbooks or having to include
lookups.
`HOME/.vault-token` is nice because a user can authenticate with the CLI
using `vault auth` and then the token will be stored in
`$HOME/.vault-token`. If we read this file, then we allow someone to do
`vault auth` "out of band" to set up Vault access.
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