fix(ci): add retry and debug logging for Windows InvalidSignatureException

[roger-zhangg]

Problem

The setup_testing_resources.py step intermittently fails (~10% of the time) on Windows GitHub runners with InvalidSignatureException when invoking the credential distribution Lambda. This never happens on Linux runners.
Example: https://github.com/aws/aws-sam-cli/actions/runs/23665127474/job/68945043702

Changes

• tests/get_testing_resources.py: Explicitly .strip() AWS credentials before passing to boto3 client to guard against any trailing whitespace/\r on Windows
• tests/setup_testing_resources.py: Added retry loop (3 attempts, 5s delay) around credential fetch, and added UTC clock logging on each attempt to help diagnose whether clock skew is the root cause

[roger-zhangg]

test integ test https://github.com/aws/aws-sam-cli/actions/runs/23669314309