br: Use BR with AWS dynamic access key

[lobshunter]

Feature Request

Support using BR with AWS dynamic access key.

Is your feature request related to a problem? Please describe:
Used dynamically generated aws access key pair to run BR:

export AWS_ACCESS_KEY_ID=xxx
export AWS_SECRET_ACCESS_KEY=xxx
export AWS_SESSION_TOKEN=xxx
tiup br backup full \
  --pd "[172.31.21.0:2379](http://172.31.21.0:2379/)" \
  --storage "s3://xxx" \
  --send-credentials-to-tikv=true \
  --s3.region "us-west-2" \
  --log-file backuptable.log

But it failed with

Error: error happen in store 1 at [172.31.25.148:20160](http://172.31.25.148:20160/): s3:<region:"us-west-2" bucket:"xxx" prefix:"xx" access_key:"x" secret_access_key:"x" force_path_style:true >  Io(Custom { kind: Other, error: "failed to put object rusoto error Request ID: None Body: <?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<Error><Code>InvalidAccessKeyId</Code><Message>The AWS Access Key Id you provided does not exist in our records.</Message><AWSAccessKeyId>x</AWSAccessKeyId><RequestId>x</RequestId><HostId>x=</HostId></Error>" }): [BR:KV:ErrKVStorage]tikv storage occur I/O error

Describe the feature you'd like:
Some way to make it success(by respecting AWS_SESSION_TOKEN or other approach).

Describe alternatives you've considered:
None

Teachability, Documentation, Adoption, Migration Strategy:
None