Retry process execution in case of ENOTDIR

[iquiw]

Copied from https://ghc.haskell.org/trac/ghc/ticket/9393

If PATH environment variable contains non directory component,
execvpe fails by ENOTDIR.

For example, if "/tmp/foo" is a regular file, the following program fails with PATH contains "/tmp/foo".

module Main where

import System.Environment
import System.Posix.Process

main :: IO ()
main = do
    env <- getEnvironment
    executeFile "echo" True [] (Just env)
    return ()

$ PATH=/tmp/foo:$PATH runghc a.hs   
a.hs: echo: executeFile: inappropriate type (Not a directory)

See for example, ​haskell/cabal#1723