nix-worker: put the pid of the caller in argv[1]

This is useful for debugging.

src/libmain/shared.cc

@@ -287,7 +287,7 @@ static void * oomHandler(size_t requested)
 
 
 int exitCode = 0;
-
+char * * argvSaved = 0;
 
 }
 
@@ -298,6 +298,8 @@ int main(int argc, char * * argv)
 {
     using namespace nix;
 
+    argvSaved = argv;
+
     /* If we're setuid, then we need to take some security precautions
        right away. */
     if (argc == 0) abort();