Allow derivations to update the build phase

So the progress bar can show

  [1/0/1 built, 0.0 MiB DL] building hello-2.10 (configuring): checking whether pread is declared without a macro... yes

src/libutil/logging.hh

@@ -31,6 +31,7 @@ typedef enum {
     resBuildLogLine = 101,
     resUntrustedPath = 102,
     resCorruptedPath = 103,
+    resSetPhase = 104,
 } ResultType;
 
 typedef uint64_t ActivityId;