Test.Utils.Paths: Fix the test data directory for Windows and Nix

When running tests under either Windows or Nix, we want the test data
directory to be relative to the current directory, rather than an
absolute path.

lib/test-utils/src/Test/Utils/Paths.hs

@@ -10,15 +10,31 @@ module Test.Utils.Paths
 
 import Prelude
 
+import Control.Monad.IO.Class
+    ( liftIO )
 import Data.FileEmbed
     ( makeRelativeToProject )
 import Language.Haskell.TH.Syntax
     ( Exp, Q, liftData )
+import System.Environment
+    ( lookupEnv )
 import System.FilePath
     ( (</>) )
 
--- | A TH function to get the test data directory. It combines the current
--- source file location and cabal file to locate the package directory in such a
--- way that works in both the package build and ghci.
+-- | A TH function to get the test data directory.
+--
+-- It combines the current source file location and cabal file to locate the
+-- package directory in such a way that works in both the stack/cabal package
+-- build and ghci.
+--
+-- For the Nix build, rather than baking in a path that starts with @/build@, it
+-- makes the test data path relative to the current directory.
 getTestData :: Q Exp
-getTestData = makeRelativeToProject ("test" </> "data") >>= liftData
+getTestData = do
+    let relPath = "test" </> "data"
+    absPath <- makeRelativeToProject relPath
+
+    -- This environment variable indicates we are building under nix.
+    nixBuildDir <- liftIO $ lookupEnv "NIX_BUILD_TOP"
+
+    liftData $ maybe absPath (const relPath) nixBuildDir