fix install_dir error

When the user's home directory and username are inconsistent, an error message stating that the directory cannot be found will appear. Directly default the installation directory to the user's home directory

webui.sh

@@ -23,7 +23,7 @@ fi
 # Install directory without trailing slash
 if [[ -z "${install_dir}" ]]
 then
-    install_dir="/home/$(whoami)"
+    install_dir="${HOME}"
 fi
 
 # Name of the subdirectory (defaults to stable-diffusion-webui)