feat: Support environment variable to always use venvs: `PDM_MULTIRUN…

…_USE_VENVS=1`

Issue #4: #4
PR #3: #3

README.md

@@ -67,6 +67,9 @@ pdm multirun -ei 3.10,3.11 pytest tests/
 pdm multirun -ei tests38,tests39 pytest tests/
 ```
 
+You can set PDM Multirun to use virtual environments by default by
+setting the `PDM_MULTIRUN_USE_VENVS` environment variable to `1`.
+
 By default, PDM Multirun reads Python versions (or venv names)
 from the `PDM_MULTIRUN_VERSIONS` environment variable.
 It is a string of `{major}.{minor}` versions (or venv names),

src/pdm_multirun/plugin.py

@@ -17,6 +17,7 @@
     from pdm.core import Core
 
 PYTHON_VERSIONS = os.getenv("PDM_MULTIRUN_VERSIONS", "").split() or [f"3.{minor}" for minor in range(8, 13)]
+USE_VENVS = os.getenv("PDM_MULTIRUN_USE_VENVS", "") == "1"
 
 
 def _comma_separated_list(value: str) -> list[str]:
@@ -47,7 +48,7 @@ def add_arguments(self, parser: argparse.ArgumentParser) -> None:  # noqa: D102
             "-e",
             "--venvs",
             action="store_true",
-            default=False,
+            default=USE_VENVS,
             help="Use virtual environments",
         )