Question: how to use cabal built for arm64 and x86_64 in mac m1

It's a question, not a bug (don't know how to set a label), sorry.

I build the project on Mac M1 which is ARM64 for 2 architectures: ARM64 and x86_64. I have 2 cabals installed:

```
cabal/
+-- arm64/
| +-- cabal
| +-- plan.json
|
+-- x86_64/
+-- cabal
+-- plan.json
```

And I use both `cabal`s, like `.../cabal/arm64/cabal ...` or `.../cabal/x86_64/cabal ...`. But I see that both `plan.json` files are different (`cabal` versions are the same though)! So, it seems that when I run particular `cabal` binary I must be sure that it uses correct `plan.json`. So, my questions are:

1. How does `cabal` determine `plan.json` that it will use?
2. How to pass the location of `plan.json` to be used by `cabal`? Some command line option for it?

Start with the two cabal invocations and their corresponding plan.json files described in the issue; check how each invocation selects its project plan. Done when the behavior and supported way to select a plan are documented for both ARM64 and x86_64 builds.

Written by the indexing model from the issue text.