Fix #1600: add end-2-end test of hix project initialization and flakes development shell

Author: yvan-sraka

.buildkite/pipeline.yml

@@ -50,3 +50,14 @@ steps:
       - nix-instantiate release.nix -A required-unstable-ghc8107-x86_64-darwin-native --show-trace --builders ''
     agents:
       system: x86_64-linux
+
+  - label: 'End-2-end test of hix project initialization and flakes development shell'
+    command:
+      - HASKELL_NIX=$(pwd)
+      - cd $(mktemp -d)
+      - nix-shell -p cabal-install --run "cabal update; cabal unpack hello"
+      - cd hello-*
+      - nix run $HASKELL_NIX#hix -- init
+      - nix develop --override-input haskellNix $HASKELL_NIX -c "cabal build"
+    agents:
+      system: x86_64-linux