Fix PATH changes not triggering REPL reconfiguration (#2015)

[mpickering]

When a user's PATH environment variable changes between Cabal commands, the environment in the REPL becomes incorrect. This occurs because during initial package configuration, the current PATH is captured via programSearchPathAsPATHVar and then stored in the programOverrideEnv field of a ConfiguredProgram. These ConfiguredPrograms are subsequently serialized into the setup-config file, effectively baking in the PATH environment from the time of configuration.

The issue manifests when PATH is updated after initial configuration. Although the solver re-runs when detecting PATH changes, the dryRunLocalPkg function examines ElaboratedConfiguredPackage and incorrectly determines that nothing has changed that would require reconfiguration. This decision is incorrect because setup-config now contains a stale reference to the original PATH value, which no longer matches the current environment.

To fix this problem, we need to store the ConfiguredPrograms directly in ElaboratedConfiguredPackage. This approach will ensure that when PATH changes, the ConfiguredPrograms will also change, properly triggering reconfiguration. While this solution will cause more recompilations when PATH changes, it guarantees that the correct environment is always used during builds and REPL sessions.

An alternative approach would be to stop baking the PATH variable into the environment of programs, but this would require more substantial changes to how Cabal manages environment variables.

Fixes #2015

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