Don't update `cabal.sandbox.config` after `configure -w`.

Use the saved values from `ConfigFlags` that were read from `dist/setup-config` instead.
    
This way `reinstallAddSourceDeps` won't reinstall deps for the wrong compiler, but the user will be forced to use `install -w` when installing inside a sandbox. This is consistent with the normal (i.e. non-sandbox) mode of operation.
    
Fixes #1333, in a way.