Better detection of secrets when redacting environment values

The check that decides whether a value looks like a secret, used to redact environment variables before they are recorded, now recognizes common token prefixes including ghp_-style GitHub tokens, github_pat_, glpat-, Slack xox tokens, sk-, pk-, AKIA, JWTs starting eyJ, ya29. and npm_, plus any string of 20 characters or more mixing letters and digits. It replaces a much narrower earlier scheme.