Delete temporary directory on successful build

With --check and the --keep-failed (-K) flag, the temporary directory
was being retained regardless of whether the build was successful and
reproducible.  This removes the temporary directory, as expected, on
a reproducible check build.

Added tests to verify that temporary build directories are not
retained unnecessarily, particularly when using --check with
--keep-failed.

src/libstore/build.cc

@@ -1680,6 +1680,7 @@ void DerivationGoal::buildDone()
         }
 
         if (buildMode == bmCheck) {
+            deleteTmpDir(true);
             done(BuildResult::Built);
             return;
         }