nix-shell: Don't act interactive in shebangs

I had observed that 'bash --rcfile' would do nothing in a
non-interactive context and cause nothing to be executed if a script
using nix-shell shebangs were run in a non-interactive context.

src/nix-build/nix-build.cc

@@ -276,6 +276,7 @@ int main(int argc, char ** argv)
                 if (n >= args.size()) {
                     throw UsageError(format("%1% requires an argument") % arg);
                 }
+                interactive = false;
                 auto interpreter = args[n];
                 auto execArgs = "";