Update a build (rename, tags)

Renames a build and/or replaces its tag set. Tags are short labels (letters, digits, dot, dash, underscore, colon, space; max 50 chars each, 10 per build) you can filter the builds list by (GET /v1/builds?tag=...) -- handy for marking CI runs as smoke, nightly, release-x and so on. Passing an empty tags array (JSON body) clears them.