Make sure that cabal-install honours --with-ghc-pkg option

[mpickering]

The cause of this bug was when ProgramDb is serialised (by caching), the UnconfiguredPrograms are discarded. cabal-install resolves this by adding back the relevant UnconfiguredPrograms in compilerProgramDb.

The cause of the bug was the order programs were added back. I have clarified the logic of the function now to:

1. Add back all unconfigured programs, and handle an explicit ghc-pkg path.
2. Then configure ghc-pkg and check it's the right version.

This makes sure that the user-supplied path is honoured in both situations.

Fixes #11373

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