[advice] add check of PREFIX for termux-widget

[Freed-Wu]

Now in android, when $SHELL exists, it will use linux config, such as ~/.config/XXX, else it will use android config.

However, for termux-widget, it will not set $SHELL.

Such as create a /data/data/com.termux/files/home/.shortcuts/test

echo $SHELL

return nothing.

Why not add a check not only for $SHELL but also for $PREFIX? the latter is all programs running in termux will set. Thanks.

https://github.com/termux/termux-widget

[gaborbernat]

PR welcome.