Change withEnv to not clear the whole env (#5281)

The way we use `withEnv` it is really intended to set a few
environment variables in an already existing environment instead of
clearing everything before. This PR changes it to do only that.

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closes #5280

daml-assistant/test/DA/Daml/Assistant/Tests.hs

@@ -33,7 +33,9 @@ import qualified Data.Conduit.Tar as Tar
 -- unix specific
 import System.PosixCompat.Files (createSymbolicLink)
 
--- | Replace all environment variables for test action, then restore them.
+-- | Set the given environment variables, then restore them.
+-- Envirnoment variables not in the given list are unmodified.
+--
 -- Avoids System.Environment.setEnv because it treats empty strings as
 -- "delete environment variable", unlike main-tester's withEnv which
 -- consequently conflates (Just "") with Nothing.
@@ -41,11 +43,7 @@ withEnv :: [(String, Maybe String)] -> IO t -> IO t
 withEnv vs m = bracket pushEnv popEnv (const m)
     where
         pushEnv :: IO [(String, Maybe String)]
-        pushEnv = do
-            oldEnv <- getEnvironment
-            let ks  = map fst vs
-                vs' = [(key, Nothing)  | (key, _) <- oldEnv, key `notElem` ks] ++ vs
-            replaceEnv vs'
+        pushEnv = replaceEnv vs
 
         popEnv :: [(String, Maybe String)] -> IO ()
         popEnv vs' = void $ replaceEnv vs'