Commit 6ca3ec8 deprecated the experimental
"--stream" option for the legacy builder, adding an error message is a client
attempted to use this feature.

However, the detection used the session-ID (`session=xxx` query parameter),
which happens to be set automatically by the CLI if it detects that the daemon
has session support. Because of this, builds fail when trying to perform them
on a daemon with the `--experimental` flag set.

This patch changes the detection to look for the `remote` query parameter, which
is set to "client-session" when using the `--stream` option with the classic
(non-Buildkit) builder.

Before this change, running `docker build` with an older (19.03 or older) cli
against a daemon with `--experimental` enabled caused an error:

    $ dockerd --experimental &
    $ docker pull docker:18.09
    $ docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock docker:18.09 sh -c 'echo "FROM scratch" | docker build -'

    Sending build context to Docker daemon  2.048kB
    Error response from daemon: experimental session with v1 builder is no longer supported, use builder version v2 (BuildKit) instead

    docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock -w /foo docker:18.09 sh -c 'echo "FROM scratch" > Dockerfile && docker build --stream .'
    Error response from daemon: experimental session with v1 builder is no longer supported, use builder version v2 (BuildKit) instead

With this patch, the error only occurs when trying to use the experimental
`--stream` option:

    $ dockerd --experimental &
    $ docker pull docker:18.09
    $ docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock docker:18.09 sh -c 'echo "FROM scratch" | docker build -'

    Step 1/1 : FROM scratch
     --->
    No image was generated. Is your Dockerfile empty?

    $ docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock -w /foo docker:18.09 sh -c 'echo "FROM scratch" > Dockerfile && docker build --stream .'

    Error response from daemon: experimental session with v1 builder is no longer supported, use builder version v2 (BuildKit) instead

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>