feat(tool): bundle python for the meson build

- add a python-win tool package that unzips the official python.org
  nuget package into the pkgroot (venv and ensurepip included)
- meson uses it to build its venv and only falls back to a system
  python, so no manual environment setup is needed

config/pkg/lib/postgresql.yml

@@ -18,6 +18,7 @@ postgresql:
     - openssl
     - zlib
   tools@windows:
+    - python-win
     - meson
     - ninja
     - winflexbison