chore(dune-project): restore symlink to .build/dune-project #256

Job	Run time
governance / Well-Known (RFC 9116 + RSR)	6s
governance / Workflow security linter	4s
governance / Code quality + docs	12s
governance / Guix primary / Nix fallback policy	5s
governance / Security policy checks	5s
governance / Language / package anti-pattern policy	6s
	38s

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