[build.webkit.org] Use passwords.json instead of environment variables.
rdar://115659344
This is good for security as well, in case of accidental leak of env variables.
Pull request: https://github.com/WebKit/WebKit/pull/18045
Committed 268277@main (6e0343960caf): <https://commits.webkit.org/268277@main> Reviewed commits have been landed. Closing PR #18045 and removing active labels.