repl: respect --print-build-logs & fix memory leak

243c0f18d[1] allowed the logger's progress bar to display during repl
builds, but startProgressBar() re-creates the entire logger from
scratch, discarding the value of printBuildLogs (and leaking the
previous logger). In this commit, we instead call logger->pause() and
logger->resume().

[1]: 243c0f18da

Change-Id: Ie734d1f638d45759d232805d7e3c2005f7dea483

src/libcmd/repl.cc

@@ -268,7 +268,7 @@ ReplExitStatus NixRepl::mainLoop()
 
     /* Stop the progress bar because it interferes with the display of
        the repl. */
-    stopProgressBar();
+    logger->pause();
 
     std::string input;
 
@@ -622,9 +622,9 @@ ProcessLineResult NixRepl::processLine(std::string line)
             // TODO: this only shows a progress bar for explicitly initiated builds,
             // not eval-time fetching or builds performed for IFD.
             // But we can't just show it everywhere, since that would erase partial output from evaluation.
-            startProgressBar();
+            logger->resume();
             Finally stopLogger([&]() {
-                stopProgressBar();
+                logger->pause();
             });
 
             state->store->buildPaths({