[libc++] Try to build both Android and Linux CI images from the same base, to avoid needing two commits on updates

Right now, the steps to update the CI images for libc++ testing are a bit awkward.

First, you need to make a change to e.g. `libcxx/utils/ci/docker/linux-builder-base.dockerfile` and land it, and get that image rebuilt via the Github action. That will provide the SHA of a new image.

Second, the `BASE_IMAGE_VERSION` of `libcxx-linux-builder` and `libcxx-android-builder` can be updated inside `libcxx/utils/ci/docker/docker-compose.yml`. Land that, and get the new non-base images to rebuild. That gets you yet another set of SHAs.

Finally, update the SHA mentioned in `libcxx/utils/ci/images/libcxx_next_runners.txt`. This is documented in `Contributing.rst`.

It would be nice to merge the first two steps such that you only have to update the images in one commit, and then go back and set the image from `libcxx_next_runners.txt` in a second commit.