build: Introduce & use pyprojectPypaBuildHook for cross builds

The way uv invokes it's environment scripts with `-I` (isolated mode) is incompatible with how nixpkgs sets up cross builds.
Nixpkgs relies on sysconfig coming through PYTHONPATH, which is ignored in isolated mode.

build/checks/default.nix

@@ -55,8 +55,9 @@ in
         }).overrideScope
           buildSystems;
     in
-    crossSet.pythonPkgsHostHost.mkVirtualEnv "cross-venv" {
+    crossSet.mkVirtualEnv "cross-venv" {
       build = [ ];
+      cffi = [ ];
     };
 
   prebuilt-wheel = pythonSet.pythonPkgsHostHost.callPackage (

build/hooks/default.nix

@@ -6,6 +6,8 @@
   lib,
   stdenv,
   hooks,
+  resolveBuildSystem,
+  pythonPkgsBuildHost,
 }:
 let
   inherit (python) pythonOnBuildForHost isPy3k;
@@ -58,6 +60,30 @@ in
         inherit (buildPackages) uv;
       };
 
+  /*
+    Build a pyproject.toml/setuptools project.
+
+    Used internally by `pyprojectHook`.
+  */
+  pyprojectPypaBuildHook =
+    callPackage
+      (
+        _:
+        makeSetupHook {
+          name = "pyproject-pypa-build-hook";
+          substitutions = {
+            inherit (pythonPkgsBuildHost) build;
+            inherit pythonInterpreter;
+          };
+          propagatedBuildInputs = pythonPkgsBuildHost.resolveBuildSystem {
+            build = [ ];
+          };
+        } ./pyproject-pypa-build-hook.sh
+      )
+      {
+        inherit (buildPackages) python;
+      };
+
   /*
     Symlink prebuilt wheel sources.
 
@@ -198,6 +224,7 @@ in
         // (lib.optionalAttrs (stdenv.buildPlatform != stdenv.hostPlatform) {
           # Uv is not yet compatible with cross installs, or at least I can't figure out the magic incantation.
           # We can use installer for cross, and still use uv for native.
+          pyprojectBuildHook = hooks.pyprojectPypaBuildHook;
           pyprojectInstallHook = hooks.pyprojectPypaInstallHook;
         })
       );

build/hooks/pyproject-pypa-build-hook.sh

@@ -0,0 +1,18 @@
+# Setup hook to use for PEP-621/setuptools builds
+echo "Sourcing pyproject-build-hook"
+
+pyprojectPypaBuildPhase() {
+  echo "Executing pyprojectPypaBuildPhase"
+  runHook preBuild
+
+  echo "Creating a wheel..."
+  env PYTHONPATH="${NIX_PYPROJECT_PYTHONPATH}:${PYTHONPATH}" @build@/bin/pyproject-build --no-isolation --outdir dist/ --wheel $pypaBuildFlags
+
+  runHook postBuild
+  echo "Finished executing pyprojectPypaBuildPhase"
+}
+
+if [ -z "${dontUsePyprojectPypaBuild-}" ] && [ -z "${buildPhase-}" ]; then
+  echo "Using pyprojectPypaBuildPhase"
+  buildPhase=pyprojectPypaBuildPhase
+fi