Make cabal update accept a freeze file

**Describe the bug**

I want CI to build. To build we want 1) a new enough index, but we also want 2) to avoid unnecessary updates. `cabal update` fits the bill for the first requirement but not the second. To get both we could use a script:

```
timestamp=$(cat ./cabal.project.freeze | grep index-state | cut -f 3 -d ' ')
repohost=$(cat ./cabal.project.freeze | grep index-state | cut -f 2 -d ' ')
cabal v2-update ${repohost},${timestamp}
```

but it would be way better to call:

```
cabal update --project-file cabal.project.freeze
```

(EDITED to remove new flag and re-use an existing flag)

Would the feature be acceptable or would modifications to the design be requested?