tests.haskell.shellFor: use srcOnly to extract pkgs

This ensures patches and Hackage revisions are applied to the base
tarball which may be necessary in order for the packages to build in the
shell environment.

pkgs/test/haskell/shellFor/default.nix

@@ -1,6 +1,7 @@
 {
   lib,
   writeText,
+  srcOnly,
   haskellPackages,
   cabal-install,
 }:
@@ -20,9 +21,9 @@
     sourceRoot=$(pwd)/scratch
     mkdir -p "$sourceRoot"
     cd "$sourceRoot"
-    tar -xf ${haskellPackages.constraints.src}
-    tar -xf ${haskellPackages.cereal.src}
-    cp ${writeText "cabal.project" "packages: constraints* cereal*"} cabal.project
+    cp -r "${srcOnly haskellPackages.constraints}" constraints
+    cp -r "${srcOnly haskellPackages.cereal}" cereal
+    cp ${writeText "cabal.project" "packages: constraints cereal"} cabal.project
   '';
   buildPhase = ''
     export HOME=$(mktemp -d)