tfenv.tests: fix the eval

Without the change the eval fails as:

    $ nix build --no-link -f. tfenv.tests
    error:
       … while evaluating the attribute 'version'
         at pkgs/by-name/tf/tfenv/package.nix:75:5:
           74|     updateScript = nix-update-script { };
           75|     tests.version = testers.testVersion {
             |     ^
           76|       package = finalAttrs.finalPackage.tfenv;

       … while evaluating the 'name' attribute of a derivation

       … while evaluating a branch condition
         at lib/strings.nix:2903:5:
         2902|     # First detect the common case of already valid strings, to speed those up
         2903|     if stringLength string <= 207 && okRegex string != null then
             |     ^
         2904|       unsafeDiscardStringContext string

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: attribute 'tfenv' missing
       at pkgs/by-name/tf/tfenv/package.nix:76:17:
           75|     tests.version = testers.testVersion {
           76|       package = finalAttrs.finalPackage.tfenv;
             |                 ^
           77|     };
       Did you mean one of env or stdenv?

pkgs/by-name/tf/tfenv/package.nix

@@ -73,7 +73,7 @@ stdenvNoCC.mkDerivation (finalAttrs: {
   passthru = {
     updateScript = nix-update-script { };
     tests.version = testers.testVersion {
-      package = finalAttrs.finalPackage.tfenv;
+      package = finalAttrs.finalPackage;
     };
   };