fix build with latest Lix

Commit 8088927b90 renamed initGC to initLibExpr.
Use the new name so that the build works again.

src/nix-eval-jobs.cc

@@ -344,7 +344,7 @@ int main(int argc, char **argv) {
 
     return handleExceptions(argv[0], [&]() {
         initNix();
-        initGC();
+        initLibExpr();
 
         myArgs.parseArgs(argv, argc);