fix(nix): clear stale wheels before pypaBuildPhase

`dist/` carried in via the source tree can contain a stale wheel from a
previous build (e.g. kpmx-1.12.1 alongside the freshly built 1.13.3).
Both wheels declare a `bin/pkgmgr` entry, so `pypaInstallPhase` hits
FileExistsError on the second install. Wipe `dist/` in `preBuild` so
only the fresh wheel is installed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flake.nix

@@ -40,6 +40,10 @@
             # Build using pyproject.toml
             format = "pyproject";
 
+            # Clear any stale wheels carried in from the source tree so
+            # pypaInstallPhase doesn't collide on bin/pkgmgr.
+            preBuild = "rm -rf dist";
+
             # Build backend requirements from [build-system]
             nativeBuildInputs = [
               pyPkgs.setuptools