Data Dir for `Paths_` modules do no get named properly with cabal multi repl

**Describe the bug**

Say I have a cabal project with two packages `a` and `b` where
- each of them has a library
- `b` depends on `a`
- `a` has a module `Paths_a`

Then, if we load `b` in `cabal repl --enable-multi-repl`, the data directory in `Paths_a` points to `~/.cabal/share` which is wrong, since there's no such directory.

When running `cabal repl --disable-multi-repl`, it works as expected.

**Expected behavior**
`getDataDir` should be the valid path, independently of whether we use cabal multi repl

**System information**
- Operating system: NixOS 25.05
- `cabal`, `ghc` versions; 3.14.1.0, 9.8.4

**Additional context**

When running with `-v3`, we can see that `cabal` *tries* to set the environment to the right value
```
Including the following environment variable overrides:
- a_datadir=/path/to/a/.
```
But it isn't getting picked up, it appears.