Addresses https://github.com/docker/setup-buildx-action/issues/45
Simple repro:
```
$ buildx create --platform linux/amd64 --use
$ buildx build - <<EOF
from scratch
EOF
```
Since https://github.com/docker/buildx/pull/370 a `platform` driver-opt was automatically inserted with the value specified by `--platform` flag on regardless of the type of driver, even though it was only used in the kubernetes driver. However, because the docker-container driver is pedantic about the options being passed, it errored out.
Another side-effect I suspect is that with the kubernetes driver it was now possible to specify the platforms in two different ways: `--driver-opt platform=...` and `--platform`.
This patch reverts completely the `platform` driver-opt and instead ensures the platforms information is passed onto the kubernetes driver via variables.
Signed-off-by: Tibor Vass <tibor@docker.com>
381dc8fb43