Change `hhvm/hhvm:latest` to use main `distroname` repository, not `distroname-4.x`

[fredemmott]

Right now, hhvm/hhvm:latest and hhvm/hhvm-proxygen:latest are aliases for the latest hhvm/hhvm:4.x-latest. Two problems:

• apt update && apt upgrade does not upgrade to the latest HHVM, only the latest for that specific x
• it's a pain to switch to a nightly if required

It should be changed to be a separate image that is not pinned to the particular x.

This likely needs changes to:

• https://github.com/hhvm/hhvm-docker/blob/master/build-and-tag.sh
• https://github.com/hhvm/packaging/blob/b32fbaa568832f0fbb70dbd6bfae1ab8228c0f8a/aws/bin/update-docker
• https://github.com/hhvm/packaging/blob/b32fbaa568832f0fbb70dbd6bfae1ab8228c0f8a/bin/promote-nightly-to-release#L99-L110

[fredemmott]

it's a pain to switch to a nightly if required

For example, it adds extra steps to attempting to use a devcontainer/github codespace to update hhast's codegen