CI and tests failing on external contributor PRs

On incoming PRs from forks, the tests fail:

  src/Data/Yaml/Config.hs:215:19: 
  1) Rollbar.Yesod.rollbarYesodMiddlewareWith, when there is an error, triggers a call to Rollbar
       uncaught exception: ErrorCall
       Could not convert to expected type: Error in $.token: parsing Token failed, expected String, but encountered Null
       CallStack (from HasCallStack):
         error, called at src/Data/Yaml/Config.hs:215:19 in yaml-0.11.11.2-72b012f81738f9aa8a0375a02116a9c2de7469091d3d770581aea04b4133eac8:Data.Yaml.Config

We found out a year ago or so that this is caused by an intentional limitation on GitHub Actions runners. Runners forbid using secrets in the CI of external PRs for security reasons (to avoid leakage and mishandling). Since we rely on a Rollbar token to run E2E tests, this means that the tests are failing.

We want to keep these E2E tests, so one potential solution would be:

• Disabling the CI in external PRs coming from forks.
• Asking external contributors to provide proof of tests passing when asking for review, or run the tests ourselves locally for external contributions, to ensure that they pass.

Start by inspecting the GitHub Actions configuration for external fork pull requests and the E2E test path; the reported failure originates at src/Data/Yaml/Config.hs:215. Confirm how the missing Rollbar token affects these tests, then establish an agreed approach so external PRs do not fail solely because secrets are unavailable while the E2E tests remain covered.