The `EARTHLY_TMP_DIR` is not actually configurable

[vladaionescu]

Although the docs say so, the EARTHLY_TMP_DIR is not actually configurable. Not using the default location may result in slow WITH DOCKER performance.

The reason this happens is that the dind path in the buildkit container is hardcoded in Earthly code: https://github.com/earthly/earthly/blob/main/earthfile2llb/withdockerrun.go#L214 (plus one other similar place).