Add pixi to excluded directories

Pixi is a recent tool that manages project-specific Conda environments.
The environments are created in a cache directory named `.pixi/` under the project root.
They should never be included in a Python distribution.
For additional discussion, see
<prefix-dev/pixi#2115 (comment)>

backend/src/hatchling/builders/constants.py

@@ -21,6 +21,8 @@
     '.pytest_cache',
     # Mypy
     '.mypy_cache',
+    # pixi
+    '.pixi',
 ))
 EXCLUDED_FILES = frozenset((
     # https://en.wikipedia.org/wiki/.DS_Store