cmd/hakurei: exec instead of fork/exec from shell

There is no reason to keep the shell process around.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/interactive/hakurei.nix

@@ -37,7 +37,7 @@
             path = "/bin/sh";
             args = [
               "-lc"
-              "sleep 1 && false"
+              "sleep 1 && exec false"
             ];
           }
         ];