allow AWS_SESSION_TOKEN to create session token

[joohae-kim]

for me, it's not easy to explaining the detail why I wanted merge the update into the main repo however, I think we can use AWS_SESSION_TOKEN variable with simple touch up please, refer following issue ticket thank you

Reference: https://github.com/fastlane-community/fastlane-plugin-s3/issues/54

[kondratk]

@joohae-kim I've tested your PR and it seems like working fine. @joshdholtz is there any chance for merging this one soon?

[paul-verkada]

@joshdholtz bump

[brunomgsantos]

@joshdholtz this feature could be really helpful for AWS Federated Logins 👍

[seanedwards]

+1, we just ran into this as well. We use SAML-federated logins to AWS, so all of our development credentials require AWS_SESSION_TOKEN