test: drop hardcoded pytest --basetemp so parallel pixi run test invocations don't race

xdist.setup_node() calls basetemp.mkdir(mode=0o700) without exist_ok=True.
With `--basetemp=pytest-temp` hardcoded in pytest.ini, the second of two
concurrent pixi-test invocations panicked with
  FileExistsError: '/var/home/tobias/src/pixi/pytest-temp'
Removing the override falls back to pytest's default
/tmp/pytest-of-USER/pytest-N/, which auto-uniquifies per invocation.

Author: hunger

pytest.ini

@@ -1,5 +1,9 @@
 [pytest]
-addopts = --basetemp=pytest-temp --timeout=600 --durations=0
+# No --basetemp override: rely on pytest's default
+# /tmp/pytest-of-<user>/pytest-N/, which auto-uniquifies per invocation.
+# Hardcoding `pytest-temp` made two concurrent `pixi run test`s race on
+# `mkdir(mode=0o700)` inside xdist.setup_node and FileExistsError out.
+addopts = --timeout=600 --durations=0
 tmp_path_retention_policy = failed
 testpaths = tests/integration_python
 markers =