Wheel build: pin pybind11 version to v2.11

This commit pins the pybind11 version used to build wheels to v2.11. 
This action was undertaken to solve a situation where, upon update to 
pybind11 v2.12, the Windows builds started failing.

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools", "wheel", "scikit-build", "cmake", "ninja", "pybind11>=2.10.1", "drjit==0.4.4"]
+requires = ["setuptools", "wheel", "scikit-build", "cmake", "ninja", "pybind11==2.11.*", "drjit==0.4.4"]
 
 [tool.pytest.ini_options]
 norecursedirs = [ "ext" ]