ci: Read env variable from os environment, not from nox (#1869)

Contains two types of changes:

- Cleanup of `DOCKER_BUILD_ARGS`: this variable is passed to the docker
build, but not used, which occasionally triggers an unnecessary rebuild.
- As consequence of `DOCKER_BUILD_ARGS` cleanup the base image has been
rebuilt. In the new image we pull a new nox version, which removes the
os environment variables from the nox session environment (refer to nox
PR wntrblm/nox#874). Therefore, such variables
need to be read from the os environment.

ci/cscs-ci.yml

@@ -34,7 +34,7 @@ stages:
     DOCKERFILE: ci/base.Dockerfile
     # change to 'always' if you want to rebuild, even if target tag exists already (if-not-exists is the default, i.e. we could also skip the variable)
     CSCS_REBUILD_POLICY: if-not-exists
-    DOCKER_BUILD_ARGS: '["CUDA_VERSION=$CUDA_VERSION", "CUPY_PACKAGE=$CUPY_PACKAGE", "CUPY_VERSION=$CUPY_VERSION", "UBUNTU_VERSION=$UBUNTU_VERSION", "PYVERSION=$PYVERSION", "CI_PROJECT_DIR=$CI_PROJECT_DIR"]'
+    DOCKER_BUILD_ARGS: '["CUDA_VERSION=$CUDA_VERSION", "CUPY_PACKAGE=$CUPY_PACKAGE", "CUPY_VERSION=$CUPY_VERSION", "UBUNTU_VERSION=$UBUNTU_VERSION", "PYVERSION=$PYVERSION"]'
 .build_baseimage_x86_64:
   extends: [.container-builder-cscs-zen2, .build_baseimage]
   variables:

noxfile.py

@@ -87,7 +87,7 @@ def test_cartesian(
         groups=["test"],
     )
 
-    num_processes = session.env.get("NUM_PROCESSES", "auto")
+    num_processes = os.environ.get("NUM_PROCESSES", "auto")
     markers = " and ".join(codegen_settings["markers"] + device_settings["markers"])
 
     session.run(
@@ -111,7 +111,7 @@ def test_examples(session: nox.Session) -> None:
     session.run(*"jupytext docs/user/next/QuickstartGuide.md --to .ipynb".split())
     session.run(*"jupytext docs/user/next/advanced/*.md --to .ipynb".split())
 
-    num_processes = session.env.get("NUM_PROCESSES", "auto")
+    num_processes = os.environ.get("NUM_PROCESSES", "auto")
     for notebook, extra_args in [
         ("docs/user/next/workshop/slides", None),
         ("docs/user/next/workshop/exercises", ["-k", "solutions"]),
@@ -131,7 +131,7 @@ def test_eve(session: nox.Session) -> None:
 
     _install_session_venv(session, groups=["test"])
 
-    num_processes = session.env.get("NUM_PROCESSES", "auto")
+    num_processes = os.environ.get("NUM_PROCESSES", "auto")
 
     session.run(
         *f"pytest --cache-clear -sv -n {num_processes}".split(),
@@ -180,7 +180,7 @@ def test_next(
         groups=groups,
     )
 
-    num_processes = session.env.get("NUM_PROCESSES", "auto")
+    num_processes = os.environ.get("NUM_PROCESSES", "auto")
     markers = " and ".join(codegen_settings["markers"] + device_settings["markers"] + mesh_markers)
 
     session.run(
@@ -211,7 +211,7 @@ def test_storage(
         session, extras=["performance", "testing", *device_settings["extras"]], groups=["test"]
     )
 
-    num_processes = session.env.get("NUM_PROCESSES", "auto")
+    num_processes = os.environ.get("NUM_PROCESSES", "auto")
     markers = " and ".join(device_settings["markers"])
 
     session.run(