Create an env variable to force `node12` actions to run on node16

We should have a way for a self hosted runner admin to force `node12` actions to run on `node16`

```
ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION=16
```

While this could be a breaking change, it's very unlikely to impact most actions. The goal is to allow runner admins more control over what node version is used, particularly in scenarios where a node version is out of date or deprecated.