[1704] Use B10_FROM_BUILD instead of B10_FROM_SOURCE (contd.)

src/lib/util/tests/run_unittests.cc

@@ -20,6 +20,6 @@ int
 main(int argc, char* argv[]) {
     ::testing::InitGoogleTest(&argc, argv);
 
-    setenv("B10_FROM_SOURCE", TEST_DATA_TOPBUILDDIR, 1);
+    setenv("B10_FROM_BUILD", TEST_DATA_TOPBUILDDIR, 1);
     return (isc::util::unittests::run_all());
 }