Due to how we pass in existing store paths via CLI arguments for the
diff and combine scripts, Nix didn't register a dependency on the store
paths properly. This meant that some of the derivations that were built,
didn't have the right store paths made available in the sandbox -
leading to all kinds of "not found" errors.
We worked around this in CI by resolving the symlinks to the nix store
beforehand. We tried to work around this locally by storing the nix
store path in BASELINE, but this didn't fully work. By explicitly
registering these store paths as dependencies, this should work across
the board - without any magic required by the caller.
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