Update actions.md

[tienthanh2509]

Add flag --with-post-file

By default, build-dir didn't execute post-files hook. In case the developer needs this hook. They can enable by the command line flag --with-post-files

distrobuilder \
        build-dir \
            --with-post-files \
            ubuntu.yaml rootfs

This PR update the document for this option, which didn't mention on any page of current document