lib: fix flake check

`builtins.currentSystem` is not available in pure eval. For this
particular test, we don't really care since it's all about generating
.drv files.

Fixes the following error:

    $ nix flake check
    warning: unknown flake output 'lib'
    error: attribute 'currentSystem' missing

           at /nix/store/8wvnlbjxlr90kq2qa6d9zjpj8rqkilr5-source/lib/tests/misc.nix:499:73:

              498|     let
              499|       deriv = derivation { name = "test"; builder = "/bin/sh"; system = builtins.currentSystem; };
                 |                                                                         ^
              500|     in {
    (use '--show-trace' to show detailed location informat

0 files changed, 0 insertions, 0 deletions