cabal-project: inject `executable-static: True` for musl hosts
Plan-to-nix's cabal-install records `--disable-executable-static` in `configure-args` for every component when the project hasn't explicitly enabled it. v1's `builder/comp-builder.nix:384` papered over this by injecting `--ghc-option=-optl=-static` (and `-optl=-pthread`) at comp-builder time — outside plan.nix's view. v1 doesn't enforce UnitId alignment with plan.json, so the post-plan flag insertion is harmless there. Under v2 the slice mirrors plan.json's `configure-args` exactly, so without something at the project level the slice links dynamically and the `tests.c-ffi.run` musl assertion (`grep "not a"` on glibc-ldd output) fails. Inject `executable-static: True` into `cabalProjectLocal` when `stdenv.hostPlatform.isMusl`. Plan-to-nix then records `--enable-executable-static`, v2's `comp-v2-builder.nix:projectConfigPragmas` round-trips it through, and cabal links the executable statically. v1 is unaffected — its post-plan `-optl=-static` injection still fires, redundant but harmless. Verified by running `tests.c-ffi.run` on the `x86_64-linux.unstable.ghc9141.musl64` jobset (commit `d027fbe4e` + this patch in a fresh worktree).