The render action does not delete environment variables that are no longer passed to it

If I set my envars as:

```
environment-variables: |
  a=1
  b=2
```

but then later change it to be:
```
environment-variables: |
  c=3
  d=4
```

I end up with all four envars defined in the ECS task.  But should only have the last two defined.