Log builder args and environment variables

Previously it only logged the builder's path, this changes it to log the
arguments at the same log level, and the environment variables at the
vomit level.

This helped me debug https://github.com/svanderburg/node2nix/issues/75

src/libstore/build/local-derivation-goal.cc

@@ -704,6 +704,9 @@ void LocalDerivationGoal::startBuilder()
 
     /* Run the builder. */
     printMsg(lvlChatty, "executing builder '%1%'", drv->builder);
+    printMsg(lvlChatty, "using builder args '%1%'", concatStringsSep(" ", drv->args));
+    for (auto & i : drv->env)
+        printMsg(lvlVomit, "setting builder env variable '%1%'='%2%'", i.first, i.second);
 
     /* Create the log file. */
     Path logFile = openLogFile();