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While debugging the odd test failure issues on macOS (x86_64) with @mpickering we came up with the following test-program (against 8.10.5), that illustrates the issue:

module Main where

import Outputable
import GHC.IO
import SysTools.Process

import Data.List ( isInfixOf, isPrefixOf )
import Data.IORef

import System.IO
import System.Process
import System.Environment
import Exception

-- See Note [Run-time linker info].
getCompilerInfo' :: String -> IO String
getCompilerInfo' pgm = do
  let -- Try to grab the info from the process output.
      parseCompilerInfo _stdo stde _exitc
        -- Regular GCC
        | any ("gcc version" `isInfixOf`) stde =
          return "GCC"
        -- Regular clang
        | any ("clang version" `isInfixOf`) stde =
          return "Clang"
        -- FreeBSD clang
        | any ("FreeBSD clang version" `isInfixOf`) stde =
          return "Clang"
        -- Xcode 5.1 clang
        | any ("Apple LLVM version 5.1" `isPrefixOf`) stde =
          return "AppleClang51"
        -- Xcode 5 clang
        | any ("Apple LLVM version" `isPrefixOf`) stde =
          return "AppleClang"
        -- Xcode 4.1 clang
        | any ("Apple clang version" `isPrefixOf`) stde =
          return "AppleClang"
         -- Unknown compiler.
        | otherwise = fail $ "invalid -v output, or compiler is unsupported (" ++ pgm ++ "): " ++ unlines stde

  -- Process the executable call
  catchIO (do
      (exitc, stdo, stde) <- do
          env <- getEnvironment
	  readCreateProcessWithExitCode ((proc pgm ["-v"]) { env = Just env }) ""
      env <- show <$> getEnvironment
      print $ length env
      -- Split the output by lines to make certain kinds
      -- of processing easier.
      parseCompilerInfo (lines stdo) (lines stde) exitc
      )
      (\err -> do
          print $ "Warning:\nCouldn't figure out C compiler information!\nMake sure you're using GNU gcc, or clang\n" ++ show err
          return "UnknownCC"
      )

main = do
  print =<< getCompilerInfo' "clang"
  main

fundamentally the issue appears to be due to the large ENV in a nix shell and readCreateProcessWithExitCode not handling large env strings. Notabley the lengh of the show on env is 29299.