fix build with latest Lix

Lix commit 4dbbd721e[1] changed the way settings are changed, removing
operator= in the process. This commit changes the places where we use
operator= to using either setDefault(), or override(). I *believe* I
have used the correct ones for each changed setting.

Fixes #13.

[1]: 4dbbd721eb

src/nix-eval-jobs.cc

@@ -350,18 +350,18 @@ int main(int argc, char **argv) {
 
         /* FIXME: The build hook in conjunction with import-from-derivation is
          * causing "unexpected EOF" during eval */
-        settings.builders = "";
+        settings.builders.setDefault("");
 
         /* Prevent access to paths outside of the Nix search path and
            to the environment. */
-        evalSettings.restrictEval = false;
+        evalSettings.restrictEval.setDefault(false);
 
         /* When building a flake, use pure evaluation (no access to
            'getEnv', 'currentSystem' etc. */
         if (myArgs.impure) {
-            evalSettings.pureEval = false;
+            evalSettings.pureEval.setDefault(false);
         } else if (myArgs.flake) {
-            evalSettings.pureEval = true;
+            evalSettings.pureEval.setDefault(true);
         }
 
         if (myArgs.releaseExpr == "")
@@ -374,7 +374,7 @@ int main(int argc, char **argv) {
         }
 
         if (myArgs.showTrace) {
-            loggerSettings.showTrace.assign(true);
+            loggerSettings.showTrace.override(true);
         }
 
         Sync<State> state_;