maintainers/…/regenerate-hackage-packages.sh: overrideable hackage2nix

Allow setting the HACKAGE2NIX environment variable to change the
hackage2nix the script uses to an absolute path to a local build. Useful
to test local changes in a realistic environment.

maintainers/scripts/haskell/regenerate-hackage-packages.sh

@@ -14,6 +14,8 @@
 
 set -euo pipefail
 
+HACKAGE2NIX="${HACKAGE2NIX:-hackage2nix}"
+
 # To prevent hackage2nix fails because of encoding.
 # See: https://github.com/NixOS/nixpkgs/pull/122023
 export LC_ALL=C.UTF-8
@@ -23,7 +25,7 @@ unpacked_hackage="$(nix-build -E "$extraction_derivation" --no-out-link)"
 config_dir=pkgs/development/haskell-modules/configuration-hackage2nix
 
 echo "Starting hackage2nix to regenerate pkgs/development/haskell-modules/hackage-packages.nix ..."
-hackage2nix \
+"$HACKAGE2NIX" \
    --hackage "$unpacked_hackage" \
    --preferred-versions <(for n in "$unpacked_hackage"/*/preferred-versions; do cat "$n"; echo; done) \
    --nixpkgs "$PWD" \