fix: use HEROKU_CLI_CHANNEL in Direwolf test trigger

[michaelmalave]

Summary

Fix the Direwolf test trigger to pass the release channel via HEROKU_CLI_CHANNEL instead of HEROKU_CLI_VERSION. The workflow has always passed a channel name (e.g. alpha), but the env var name HEROKU_CLI_VERSION caused the Direwolf test helper to treat it as a semver and run heroku update --version alpha`, which fails and skips all tests.

• Replace HEROKU_CLI_VERSION with HEROKU_CLI_CHANNEL in the workflow env and runner script
• Update the Direwolf API payload JSON key from HEROKU_CLI_VERSION to HEROKU_CLI_CHANNEL

Type of Change

• fix: Bug fix or issue (patch semvar update)
• feat: Introduces a new feature to the codebase (minor semvar update)
• perf: Performance improvement
• docs: Documentation only changes
• tests: Adding missing tests or correcting existing tests
• chore: Code cleanup tasks, dependency updates, or other changes

Verification

CI Passes

Additional Context

• Breaking: none
• Risk: low; variable-name alignment only
• Follow-up: verify the downstream Direwolf suite expects HEROKU_CLI_CHANNEL (and not the old key) in production runs

Related Issue

GUS work item: W-22934969

[tlowrimore-heroku]

LGTM