Multiple tests fail on Mac with "Error: DEVELOPER_DIR not set"

First I should mention that I needed to upgrade my `rules_haskell` version for the project to build at all. My `WORKSPACE` looks like:

```
git_repository(
name = "rules_haskell",
remote = "https://github.com/tweag/rules_haskell",
commit = "7abaefdde3d0a29bfe15e02be4ad2a0bd4484600",
shallow_since = "1626246473 +0000",
)
```

When building on my Mac I ran into [this issue](https://github.com/tweag/rules_haskell#macos-error-developer_dir-not-set), which I fixed with `export BAZEL_USE_CPP_ONLY_TOOLCHAIN=1` in my shell. But when I ran the tests under `//hrepl/tests` (via `run_tests.sh`) I noticed that many of them (but not all) failed with the same error in the logs. (Sorry, I don't have the complete list handy, but `atomic_primops_test` was one of them.)

It seems that the `BAZEL_USE_CPP_ONLY_TOOLCHAIN` environment variable doesn't make its way into the builds that `hrepl` itself is doing (which makes sense, since it should try to be hermetic). Unfortunately the only workaround I could find was to edit `ReplTestLib.hs` and manually add it to the constructed environment. This clearly isn't a good permanent solution since it's Mac-specific.