Support AWS session credentials

[skeggse]

We use session credentials from the result of an sts:AssumeRole call (via the configure-aws-credentials action). These credentials take three parts rather than the two of static credentials: AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN. bazel-remote does not seem to consume the AWS_SESSION_TOKEN environment variable, so it tries to interpret these credentials as static credentials:

WARNING: Remote Cache: io.grpc.StatusRuntimeException: UNKNOWN: The AWS Access Key Id you provided does not exist in our records.

Ideally, this common form of expressing AWS credentials would be supported by bazel-remote.

References:

• seems very related to Bazel-remote doesn't support Security Token Service (STS) authentication for S3 #490, but that never concerned itself with AWS_SESSION_TOKEN - just with the profile option

There's probably a workaround for this by using a configuration file and a profile flag, but

[pauldraper]

A workaround that should work: #493 (comment)