Add OpenWRT download cache

Some server hosting firmware binaries crashed[1] and no mirrors are
available in OpenWRT build system. This causes builds to fail for some
architectures.

To avoid these kinds of failures in the future, add a Git repository
where build jobs will commit their downloaded artifacts. Subsequent
build jobs will check out this repository and use it as their download
directory.

[1] https://github.com/openwrt/openwrt/pull/3573#issuecomment-723530425