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

Use the 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 mode of operation.

Fixes #1333, in a way.
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Merge pull request #1334 from 23Skidoo/no-with-compiler-sandbox-config

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