Add package with file command

[ajelinski]

The file command is used during building of some packages (e.g. gcc/newlib).

It's present in all Ubuntu virtual environments on GitHub Actions but it doesn't seem to be available in macOS and Windows virtual environments.
It's not available by default in Ubuntu and Debian Docker images.

Anaconda's main (AKA defaults) channel doesn't contain that tool too (only conda-forge) so let's add it to the litex-hub channel.

GH repository: https://github.com/file/file