test: update TestFnContainerEnvVars expectation for quote preservation

After switching Resource.AsYAML() to use RNode.MustString(), YAML output
now preserves original quotes from the source YAML. The template uses %q
which generates quoted values, and these quotes are now preserved in the
output. Update the test expectation accordingly.

This is a follow-up to the MustString() migration in PR kubernetes-sigs#6046.

Author: Rindrics

api/krusty/fnplugin_test.go

@@ -735,11 +735,11 @@ template: |
 	actual, err := m.AsYaml()
 	require.NoError(t, err)
 	assert.Equal(t, `apiVersion: v1
-data:
-  value: value
 kind: ConfigMap
 metadata:
   name: env
+data:
+  value: "value"
 `, string(actual))
 }