When running the AWS module it will parse the AWS config found in
`~/.aws/config` to get the region. This means that tests that expect no
region to be set will fail if there exists a default profile with a
region set, which is probably true for most AWS users. To avoid this
have set the AWS tests that depend on the non-existance of a
`.aws/config` to be ignored.
This creates a custom logger for the log crate which logs everything to a file (/tmp/starship/session_$STARSHIP_SESSION_KEY.log) and it logs everything above Warn to stderr, but only if the log file does not contain the line that should be logged resulting in an error or warning to be only logged at the first starship invocation after opening the shell.
* Use AWS_VAULT as the profile if set
[aws-vault](https://github.com/99designs/aws-vault) sets
the `AWS_VAULT` env var instead of `AWS_PROFILE` when an
aws-vault session is active.
This PR adds support for reading the AWS profile value from
the `AWS_VAULT` env var, giving priority to `AWS_VAULT` if both
it and `AWS_PROFILE` are set.
* Update docs for AWS_VAULT
