In build action, viewer_channel should be determined from branch name

[vir-linden]

Currently we use a named tag both to trigger the "release" step and to determine the viewer_channel name. This means that any builds that don't get a release step are Second Life Test channel, even if they are made from a maint RC branch. It would be more consistent and considerably simpler to just use the branch name to determine the channel

• main and anything starting with release/ are in Second Life Release
• project/foo_bar is Second Life Project Foo Bar
• anything else is Second Life Test