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

lib/tests/misc.nix

@@ -496,7 +496,7 @@ runTests {
 
   testToPretty =
     let
-      deriv = derivation { name = "test"; builder = "/bin/sh"; system = builtins.currentSystem; };
+      deriv = derivation { name = "test"; builder = "/bin/sh"; system = "aarch64-linux"; };
     in {
     expr = mapAttrs (const (generators.toPretty { multiline = false; })) rec {
       int = 42;