remove "CHANNEL=test" check for buildx, install on 20.10 as well

[thaJeztah]

remove "CHANNEL=test" check for docker-buildx-plugin

Now that docker 22.06 and the docker-buildx-plugin is available in the stable channel, we no longer need this check.

NOTE: technically, it's possible to install the buildx plugin on older versions as well (it will replace the bundled buildx plugin), so we could consider changing the version to (e.g.) 20.10, same as we did for docker compose.

install docker-buildx-plugin on version 20.10 as well, to allow upgrading

The docker-buildx-plugin package can replace the version of buildx that's bundled in the docker-ce-cli packages. This allows for buildx to be updated to the current version, even if it was originally installed from the bundled version.

Some testing shows that this works well, and that upgrading the docker-ce-cli package won't revert the change, and continues using the buildx version from the separate package.

Here's some steps to illustrate that scenario:

# remove any installed docker packages
apt-get purge -y docker-ce docker-ce-cli containerd.io docker-compose-plugin docker-scan-plugin docker-buildx-plugin docker-ce-rootless-extras

# stop the docker.socket service (it blocks a clean reinstall after an uninstall)
systemctl stop docker.socket

# install docker 20.10.14 (from the test channel)
curl -fsSL https://test.docker.com | VERSION=20.10.14 sh

# verify that the installed version of buildx is the bundled (`-docker`) version:
docker buildx version
github.com/docker/buildx v0.8.1-docker 5fac64c2c49dae1320f2b51f1a899ca451935554

# install the separate `docker-buildx-plugin` package
apt-get install -y docker-buildx-plugin

# verify that it replaced the bundled version
docker buildx version
github.com/docker/buildx v0.8.2

# upgrade to docker 20.10.17
curl -fsSL https://test.docker.com | VERSION=20.10.17 sh

# verify that updating didn't downgrade the buildx plugin, or revert it to the "included" buildx version:
docker buildx version
github.com/docker/buildx v0.8.2

[thaJeztah]

/cc @tianon @tonistiigi @chris-crone

[thaJeztah]

CI is expected to fail currently

[thaJeztah]

Thinking of this; I don't think I tested this yet on an rpm distro, so will need to try that as well