fix(app): get_arg_or_config gets environment correctly

Previously, if a value was in the passed Namespace, it would return
that. The correct behaviour however is to check the environment for
the value and return that instead.

craft_application/application.py

@@ -505,7 +505,10 @@ def get_arg_or_config(
         :param item: the name of the namespace or config item.
         :returns: the requested value.
         """
-        return getattr(parsed_args, item, self.services.config.get(item))
+        arg_value = getattr(parsed_args, item, None)
+        if arg_value is not None:
+            return arg_value
+        return self.services.config.get(item)
 
     def _run_inner(self) -> int:
         """Actual run implementation."""

tests/unit/test_application.py

@@ -614,6 +614,38 @@ def test_run_managed_empty_plan(app, fake_project):
         app.run_managed(None, None)
 
 
+@pytest.mark.parametrize(
+    ("parsed_args", "environ", "item", "expected"),
+    [
+        (argparse.Namespace(), {}, "build_for", None),
+        (argparse.Namespace(build_for=None), {}, "build_for", None),
+        (
+            argparse.Namespace(build_for=None),
+            {"CRAFT_BUILD_FOR": "arm64"},
+            "build_for",
+            "arm64",
+        ),
+        (
+            argparse.Namespace(build_for=None),
+            {"TESTCRAFT_BUILD_FOR": "arm64"},
+            "build_for",
+            "arm64",
+        ),
+        (
+            argparse.Namespace(build_for="riscv64"),
+            {"TESTCRAFT_BUILD_FOR": "arm64"},
+            "build_for",
+            "riscv64",
+        ),
+    ],
+)
+def test_get_arg_or_config(monkeypatch, app, parsed_args, environ, item, expected):
+    for var, content in environ.items():
+        monkeypatch.setenv(var, content)
+
+    assert app.get_arg_or_config(parsed_args, item) == expected
+
+
 @pytest.mark.parametrize(
     ("managed", "error", "exit_code", "message"),
     [