CI: fix Bugfix validation master-build lookup for the new S3 prefix scheme

Follow-up for https://github.com/ClickHouse/ClickHouse/pull/110081, which moved build artifacts under the normalized workflow name: `REFs/master/<sha>/masterci/build_<bt>/clickhouse`. `find_master_builds` still probed the old `REFs/master/<sha>/build_<bt>/clickhouse` path, so once every commit in the `master_commits` window had only new-layout builds, both `Bugfix validation (functional tests)` jobs started failing with `AssertionError: Could not find master builds in S3` on every pull request, for example https://github.com/ClickHouse/ClickHouse/pull/116057.