report,ui: name the tee log by flow kind

BuildTarget::source_only carried two states for what is a three-way
distinction now that pkh test exists: replace it with a TargetKind
(Source/Binary/Test) and derive the tee log prefix (build-/deb-/test-)
from it, so test runs get their own log family instead of borrowing
the deb one.

src/build/mod.rs

@@ -314,7 +314,7 @@ pub fn run_source_build(
             entry.version.full(),
             entry.distribution
         ),
-        source_only: true,
+        kind: crate::report::TargetKind::Source,
         tee_log: true,
     });