`cabal-validate`: Build products are not shared with `cabal build`

`validate.sh` creates a custom `dist-newstyle` directory, but won't reuse build products from `cabal build`, even for the same compiler, forcing pointless rebuilds.

**Proposal:** If no `--with-compiler` is passed and the `dist-newstyle-validate-*` directory doesn't exist, `cabal-validate` should copy `dist-newstyle` to `dist-newstyle-validate-*` as a starting point. Should this be the default?