installer: replace the current shell with the new ZSH instance

This replaces the currently running process with the new one using `exec`
instead of creating a new process. This way, when the user `exit`s out of
the new shell it will not pop them back into the shell from which ohmyzsh
was installed from.

tools/install.sh

@@ -149,7 +149,7 @@ main() {
 	EOF
 	printf "$NORMAL"
 
-	env zsh -l
+	exec zsh -l
 }
 
 main