BUILD: add stub_shebangs to py_runtimes for NixOS

This is needed as otherwise NixOS bazel will happily use hardcoded
/nix/store paths here.

Change-Id: Ia32268fd6cb249c05c0ec8f550d761444616adbf
Reviewed-on: https://review.monogon.dev/c/monogon/+/1147
Reviewed-by: Leopold Schabel <leo@monogon.tech>
Tested-by: Leopold Schabel <leo@monogon.tech>
diff --git a/BUILD b/BUILD
index 06407d2..eb77d5e 100644
--- a/BUILD
+++ b/BUILD
@@ -33,12 +33,16 @@
     name = "host_python3",
     interpreter_path = "/usr/bin/python3",
     python_version = "PY3",
+    # Necessary on NixOS, as the nixpkgs-packaged Bazel has this overriden to
+    # the local /nix/store.
+    stub_shebang = "#!/usr/bin/python3",
 )
 
 py_runtime(
     name = "host_python2",
     interpreter_path = "/usr/bin/python2",
     python_version = "PY2",
+    stub_shebang = "#!/usr/bin/python2",
 )
 
 py_runtime_pair(