Revert "tests.haskell.cabalSdist.localFromCabalSdist: prevent unnecessary rebuilds"

This reverts commit 81c06bc609.

Reason for revert: This change breaks the
`tests.haskell.cabalSdist.assumptionLocalHasDirectReference` test which
relies on checking for the test source store path in the resulting
derivation files. 81c06bc609 did not
account for this in the change (though it should be possible).

pkgs/test/haskell/cabalSdist/local/generated.nix

@@ -3,14 +3,7 @@
 mkDerivation {
   pname = "local";
   version = "0.1.0.0";
-  src = lib.fileset.toSource {
-    root = ./.;
-    fileset = lib.fileset.unions [
-      ./app
-      ./CHANGELOG.md
-      ./local.cabal
-    ];
-  };
+  src = ./.;
   isLibrary = false;
   isExecutable = true;
   executableHaskellDepends = [ base ];