Use configured compiler for cabal sandbox hc-pkg
For most commands (e.g. `cabal install`), the `package-db` field in `cabal.sandbox.config` is ignored, and the path is reconstructed from the `prefix` and current compiler instead. This is arguably the right behaviour, because the right package DB depends on the arch/compiler, so it doesn't make sense to specify only one. This commit makes `cabal sandbox hc-pkg` behave similarly, to avoid invoking ghc-pkg on an incompatible package DB (fixes #1935). Moreover, the compiler version to use is now picked up from the most recently configured compiler, if any. Otherwise, the global default compiler is used, as before.
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