ci: Add macos runner

[evnu]

Add a CI runner for macOS. On macos, we need to run rustc separately for each target in the Cargo.toml. The reason is that we currently pass extra linker arguments to rustc. This is not possible if multiple targets need to be build.

[evnu]

Compilation on macOS currently fails for the tests, as we pass extra arguments:

error: extra arguments to `rustc` can only be passed to one target, consider filtering
the package by passing, e.g., `--lib` or `--bin NAME` to specify a single target

== Compilation error in file lib/rustler_test.ex ==
** (RuntimeError) Rust NIF compile error (rustc exit code 101)
    (rustler 0.22.2) lib/rustler/compiler.ex:36: Rustler.Compiler.compile_crate/2
    lib/rustler_test.ex:6: (module)
    (stdlib 3.16.1) erl_eval.erl:685: :erl_eval.do_apply/6

For this to work, we need to call cargo build/cargo rustc separately for each target.

[evnu]

Compilation on macOS currently fails for the tests, as we pass extra arguments:

I work around that with 65d4171. For macOS, each target (bin or lib) is compiled separately. This feels kind-of hacky.

[evnu]

Maybe someone from @rusterlium/core can give some early feedback here.

[evnu]

Maybe we can simplify this PR by picking up #235 again: If we do not set the extra linker arguments on macos anymore, we can run cargo build directly and don't need to shell out for each lib/bin.

[evnu]

I rebased on #439 to check if the runner works on that branch.

[evnu]

As this no longer requires changes to Rustler.Compiler and simply adds a runner, I will merge this.