nix-tools-0.4.3

static-nix-tools: patch cabal-install to pin unit-id OS via env var

`hashedInstalledPackageId` selects between Long / Short / VeryShort
unit-id formats based on `buildOS` — the OS where cabal-install is
currently executing.  For haskell.nix that's the *eval* platform of
the plan-nix derivation, not the *build* platform where cabal will
later actually do the compile.  When the two differ (e.g. evaluating
on Darwin while building x86_64-linux derivations), plan-nix unit-ids
diverge from the unit-ids slice cabal v2-build computes — every slice
then tries to rebuild every dep from source.

Add a `CABAL_INSTALLED_PACKAGE_ID_OS` env var that overrides
`buildOS` for unit-id format selection.  haskell.nix sets it to the
build platform's OS when invoking `make-install-plan`.