Disallow uploading pipelines containing interpolated secrets by default
Further to #1589, pipeline secret redaction should become default in Agent v4.
This PR makes it so that default behaviour is to disallow pipeline uploads containing interpolations of potentially secret environment variables. We add flag to buildkite-agent pipeline upload to allow uploading pipelines with these secrets, but note in the CLI help and the log output that this behaviour is insecure.
We won't merge this until we release Agent v4
pipeline secret redaction should become default in Agent v4.
I don't know if this issue is complete, but there's an historical attempt to list the breaking changes we're saving for v4 over in #1391
@yob already in there :)