Allow overriding default "find" binary

I had to do this because the default find utility on my Synology NAS is a busybox function that is missing `-iregex` - installing findutils via the package manager ipkg doesn't override the default and creates a `findutils-find` executable instead.

I'm note sure if you want this in the code since this probably won't affect many users, just close the pull request if you don't want it. It is mostly copy and paste from the `UNRAR_BINARIES` stuff.