nixos/tests: fix wrong inherit that passes on the nodes attrs

The hydra tarball step would fail due to the nodes attribute not being
properly inherited. Since we can't execute all the tests and release
steps locally anymore (thanks to the JSONification and faster hydra
eval) these errors will probably keep in appearing.

This is hopefully the last of those introduced by me test runner
refactoring.

Error was seen on hydra (https://hydra.nixos.org/build/129282411):
> unpacking sources
> unpacking source archive /nix/store/bp95x52h6nv3j8apxrryyj2rviw682k1-source
> source root is source
> patching sources
> autoconfPhase
> No bootstrap, bootstrap.sh, configure.in or configure.ac. Assuming this is not an GNU Autotools package.
> configuring
> release name is nixpkgs-21.03pre249116.1088f059401
> git-revision is 1088f05
> building
> no Makefile, doing nothing
> running tests
> warning: you did not specify '--add-root'; the result might be removed by the garbage collector
> warning: you did not specify '--add-root'; the result might be removed by the garbage collector
> checking Nixpkgs on i686-linux
> checking Nixpkgs on x86_64-linux
> checking Nixpkgs on x86_64-darwin
> checking eval-release.nix
> trace: `mkStrict' is obsolete; use `mkOverride 0' instead.
> trace: `lib.nixpkgsVersion` is deprecated, use `lib.version` instead!
> trace: warning: lib.readPathsFromFile is deprecated, use a list instead
> trace: Warning: `showVal` is deprecated and will be removed in the next release, please use `traceSeqN`
> trace: lib.zip is deprecated, use lib.zipAttrsWith instead
> checking find-tarballs.nix
> trace: `mkStrict' is obsolete; use `mkOverride 0' instead.
> trace: `lib.nixpkgsVersion` is deprecated, use `lib.version` instead!
> trace: warning: lib.readPathsFromFile is deprecated, use a list instead
> trace: Warning: `showVal` is deprecated and will be removed in the next release, please use `traceSeqN`
> trace: lib.zip is deprecated, use lib.zipAttrsWith instead
> error: while evaluating anonymous function at /build/source/maintainers/scripts/find-tarballs.nix:6:1, called from undefined position:
> while evaluating 'operator' at /build/source/maintainers/scripts/find-tarballs.nix:27:16, called from undefined position:
> while evaluating 'immediateDependenciesOf' at /build/source/maintainers/scripts/find-tarballs.nix:39:29, called from /build/source/maintainers/scripts/find-tarballs.nix:27:44:
> while evaluating anonymous function at /build/source/lib/attrsets.nix:234:10, called from undefined position:
> while evaluating anonymous function at /build/source/maintainers/scripts/find-tarballs.nix:40:37, called from /build/source/lib/attrsets.nix:234:16:
> while evaluating 'derivationsIn' at /build/source/maintainers/scripts/find-tarballs.nix:42:19, called from /build/source/maintainers/scripts/find-tarballs.nix:40:40:
> while evaluating 'canEval' at /build/source/maintainers/scripts/find-tarballs.nix:48:13, called from /build/source/maintainers/scripts/find-tarballs.nix:43:9:
> while evaluating the attribute 'nodes' at /build/source/nixos/lib/testing-python.nix:195:23:
> attribute 'nodes' missing, at /build/source/nixos/lib/testing-python.nix:193:16
> build time elapsed:  0m0.122s 0m0.043s 17m51.526s 0m56.668s
> builder for '/nix/store/96rk3c74vrk6m3snm7n6jhis3j640pn4-nixpkgs-tarball-21.03pre249116.1088f059401.drv' failed with exit code 1

nixos/lib/testing-python.nix

@@ -192,7 +192,7 @@ rec {
     else
       test // {
         inherit test driver driverInteractive;
-        inherit (test) nodes;
+        inherit (driver) nodes;
       };
 
   runInMachine =