The `Nix CI` workflow's `Assume the AWS role` step of `build-and-test` fails on forks

[Josh Soref]

Describe the bug

The Nix CI workflow's Assume the AWS role step of build-and-test fails on forks:
https://github.com/jsoref/hhvm/runs/7253278295?check_suite_focus=true

Standalone code, or other way to reproduce the problem

Steps to reproduce the behavior:

1. Fork this repository
2. Wait a day or two (so that the HEAD of the fork will be behind this repo)
3. Use GitHub's fetch upstream feature
   
4. Wait for workflow to fail.

Expected behavior

If the hhvm vm commit that was the head in this repo's workflow passes, it should pass in a fork (or not run, if it really can't run, but it looks like most of nix can run).

Actual behavior

Run aws-actions/configure-aws-credentials@v1
Error: Couldn't retrieve verification key from your identity provider,  please reference AssumeRoleWithWebIdentity documentation for requirements

Environment

• Operating system
  • Environment: macos-11
• Installation method
  • 'built from source',
• HHVM Version
  • jsoref@ffe2ab9 = ffe2ab9

Additional context
Add any other context about the problem here.