ExistsSync fixes

[bretambrose]

• Fix some deprecation warnings (Fixing warning: "Passing invalid argument types to fs.existsSync is eprecated" #434)
• Update CI use action-sourced session-based credentials
• Update integration tests to only use CRT resources
• Removed some fields from debug logging

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mpatfield]

Thanks for your persistence @bretambrose! I didn’t intend for such a simple change to cause you so much trouble! 😬

[bretambrose]

Change was great. But when CI doesn't get run for over a year, things break. It was good to find and fix them. Thanks again.