Darc add-build-to-channel has an overly generous substring match

Migrated from https://github.com/dotnet/core-eng/issues/10676

@MattGal wrote:

As noted in adhoc-testing for https://github.com/dotnet/arcade-services/pull/1375 , if you do something silly like:

`Darc.exe add-build-to-channel --id 60 --channel 79`

You might expect this to be the channel id (though id is obviously not in the param name). Unexpectedly though, I had a test channel randomly named `923602799` this got added to due to the 79 in the name.

Thoughts here:

- Channel should probably always be an explicit match
- Add via Channel id might be a nice feature add; it's much easier to remember a short # for channel than the correct name for it, so users might want to actually have this option.