ci: upgrade openwrt/gh-action-sdk workflow to v7

ci: upgrade openwrt/gh-action-sdk workflow to v7

With the change of the Docker container not including pre-built
binaries [1], the previous version of the workflow fails as it still
expects those to be present in the upstream container while the new
versions runs a file called setup.sh if present to configure the
container.

By upgrading openwrt/gh-action-sdk workflow to v7, we make sure that
setup.sh is run and configures the container [2].

[1] https://github.com/openwrt/docker/commit/9b55784b18f8d2c684aac4dd21a224320bb9b9ce
[2] https://github.com/openwrt/gh-action-sdk/commit/0c00b28c11bed5c7697e807d4cb8ef176ca688ef

Signed-off-by: Daniel Nilsson <dannil+github@protonmail.com>