Enable Windows rbe_repo tests in CI

This issue is for tracking enabling rbe_repo tests in CI. Currently these are disabled as we don't have a CI environment that allows us to pull Windows Docker images. Once we do, we should enable the Windows tests in CI and potentially add some integration testing to add to Windows coverage.

In addition, we should change the container image used by the WORKSPACE rbe_autoconfig rules used for tests to one owned by the bazel-toolchains maintainers. Currently these rules use an image from the Envoy project Windows build toolchain.

These sections should be updated once we have a place to copy this container image:
https://github.com/bazelbuild/bazel-toolchains/blob/834b89b3f608a9aa4c212e7ee4a05b323b980a85/WORKSPACE#L275-L277
https://github.com/bazelbuild/bazel-toolchains/blob/834b89b3f608a9aa4c212e7ee4a05b323b980a85/WORKSPACE#L424-L431
https://github.com/bazelbuild/bazel-toolchains/blob/834b89b3f608a9aa4c212e7ee4a05b323b980a85/tests/rbe_repo/BUILD#L356

We can probably copy the latest image from here to a bazel-toolchains owned repo in GCR: https://hub.docker.com/r/envoyproxy/envoy-build-windows2019/tags

Review the WORKSPACE sections at lines 275-277 and 424-431, plus tests/rbe_repo/BUILD around line 356, to understand the current Windows image references and disabled tests. Confirm the available CI environment and the Docker image source first. Done means the Windows rbe_repo tests run in CI and the referenced image is owned by the bazel-toolchains maintainers; additional integration coverage is optional.

Written by the indexing model from the issue text.