BuildKit should warn about unconsumed build-args

relates to https://github.com/docker/cli/issues/441#issuecomment-841391407

The classic builder printed a warning if a build-arg was passed, but not used in the Dockerfile. BuildKit does not have an equivalent for this

Without BuildKit:

DOCKER_BUILDKIT=0 docker build --build-arg FOO=bar -<<EOF
FROM busybox
RUN echo hello
EOF

Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM busybox
 ---> 388056c9a683
Step 2/2 : RUN echo hello
 ---> Running in 0ff64e1edd4b
hello
Removing intermediate container 0ff64e1edd4b
 ---> 26859e806085
[Warning] One or more build-args [FOO] were not consumed
Successfully built 26859e806085

With BuildKit:

DOCKER_BUILDKIT=1 docker build --build-arg FOO=bar -<<EOF
FROM busybox
RUN echo hello
EOF

[+] Building 0.5s (6/6) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                                                                   0.1s
 => => transferring dockerfile: 70B                                                                                                                                                                    0.0s
 => [internal] load .dockerignore                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/library/busybox:latest                                                                                                                                      0.0s
 => CACHED [1/2] FROM docker.io/library/busybox                                                                                                                                                        0.0s
 => [2/2] RUN echo hello                                                                                                                                                                               0.3s
 => exporting to image                                                                                                                                                                                 0.0s
 => => exporting layers                                                                                                                                                                                0.0s
 => => writing image sha256:af09f7baeb292afd37bf948398584d8d8299ede11aae4bf0654f038e082001fa

Start by tracing how BuildKit processes build arguments during a Dockerfile build, using the issue's classic-builder and BuildKit examples as the behavioral reference. Done means an unused argument produces an equivalent warning during a BuildKit build, while a consumed argument does not.