test: pin test-env system identity and core-plugins path

PropertyMap tests went through SetPermissions, which looks up the `dokku` user/group; the fresh `golang:1.26.2` container used by `go-test-plugin-in-docker` has neither, so the chown failed with `unknown group dokku`. Override `DOKKU_SYSTEM_USER` and `DOKKU_SYSTEM_GROUP` to `root` in the property and chart-migration test setups so the chown targets an identity that exists everywhere. The config tests also called `plugn trigger post-config-update` without `PLUGIN_CORE_AVAILABLE_PATH` set, so the hook tried to source `/common/functions` and logged a noisy warning on every test; point the env var at the canonical `/var/lib/dokku/core-plugins/available` location.

plugins/common/properties_test.go

@@ -19,6 +19,8 @@ func setupPropertyMapTest(t *testing.T) {
 	RegisterTestingT(t)
 	Expect(setupTests()).To(Succeed())
 	t.Setenv("DOKKU_LIB_ROOT", t.TempDir())
+	t.Setenv("DOKKU_SYSTEM_USER", "root")
+	t.Setenv("DOKKU_SYSTEM_GROUP", "root")
 	Expect(PropertySetup(propertyTestPlugin)).To(Succeed())
 }
 

plugins/config/config_test.go

@@ -25,7 +25,11 @@ func setupTests() (err error) {
 		return err
 	}
 
-	return os.Setenv("PLUGIN_ENABLED_PATH", "/var/lib/dokku/plugins/enabled")
+	if err := os.Setenv("PLUGIN_ENABLED_PATH", "/var/lib/dokku/plugins/enabled"); err != nil {
+		return err
+	}
+
+	return os.Setenv("PLUGIN_CORE_AVAILABLE_PATH", "/var/lib/dokku/core-plugins/available")
 }
 
 func setupTestApp() (err error) {

plugins/scheduler-k3s/triggers_test.go

@@ -13,6 +13,8 @@ func setupChartMigrationTest(t *testing.T) {
 	t.Setenv("PLUGIN_PATH", "/var/lib/dokku/plugins")
 	t.Setenv("PLUGIN_ENABLED_PATH", "/var/lib/dokku/plugins/enabled")
 	t.Setenv("DOKKU_LIB_ROOT", t.TempDir())
+	t.Setenv("DOKKU_SYSTEM_USER", "root")
+	t.Setenv("DOKKU_SYSTEM_GROUP", "root")
 	Expect(common.PropertySetup("scheduler-k3s")).To(Succeed())
 }