fix: don't crash on startup on macOS

This is caused, through several layers of absurdity, by runtime type
information of Nix things being invisible due to -fvisibility=hidden
inside n-e-j.

We don't have any idea why n-e-j has -fvisibility=hidden, since blame
says it's from the initial commit. It is plausible that it was some
ill-advised optimization but it's not sound.

The crash is caused by dynamic_cast<RootArgs *>(MyArgs *) failing, which
is in turn caused by the RTTI being invisible.

See: https://www.qt.io/blog/quality-assurance/one-way-dynamic_cast-across-library-boundaries-can-fail-and-how-to-fix-it
Fixes: lix-project/nix-eval-jobs#2

src/eval-args.hh

@@ -12,7 +12,7 @@
 
 class MyArgs : virtual public nix::MixEvalArgs,
                virtual public nix::MixCommonArgs,
-               virtual nix::RootArgs {
+               virtual public nix::RootArgs {
   public:
     std::string releaseExpr;
     nix::Path gcRootsDir;

src/meson.build

@@ -31,4 +31,4 @@ executable('nix-eval-jobs', src,
              threads_dep
            ],
            install: true,
-           cpp_args: ['-std=c++2a', '-fvisibility=hidden', '--include', 'autotools-config.h'])
+           cpp_args: ['-std=c++2a', '--include', 'autotools-config.h'])