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

[9999years]

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?