`hls-gadt-plugin` test failures in Gentoo Linux

I'm getting quite a few test failures for `hls-gadt-plugin` that look more-or-less like this:

```
SingleDeriving: FAIL (2.78s)
Test output was different from 'test/testdata/SingleDeriving.expected.hs'. Output of ["git","-c","core.fileMode=false","diff","--no-index","--text","--exit-code","test/testdata/SingleDeriving.expected.hs","
/tmp/SingleDeriving.expected1634647-14.actual"]:
diff --git a/test/testdata/SingleDeriving.expected.hs b/tmp/SingleDeriving.expected1634647-14.actual
index 5a8d088..414b208 100644
--- a/test/testdata/SingleDeriving.expected.hs
+++ b/tmp/SingleDeriving.expected1634647-14.actual
@@ -1,3 +1,4 @@
+{-# LANGUAGE GADTs #-}
module SingleDeriving where

data Foo a b where

Use -p '$0=="GADT.SingleDeriving"' to rerun this test only.
```

```
16 out of 19 tests failed (13.13s)
```

It seems like `{-# LANGUAGE GADTs #-}` is being added to the source code, which is not what is expected. These failures are happening inside Gentoo's sandbox and also when the tests are run outside of it.

---

Full test output is [here](https://gist.github.com/hololeap/7b24108a6b46b8055a03029a4dff3e40)

---

### Your environment

GHC 9.0.2 on Gentoo Linux. All packages are installed through the system package manager.

---

Other `gentoo-haskell`-related test issues:

- https://github.com/haskell/haskell-language-server/issues/3126
- https://github.com/haskell/haskell-language-server/issues/3217