The recommendation (coming from rust-lang/rust#130062) for Linux hosts
is using C compiler driver as a linker, which is able to find
system-wide libraries. Using linker binaries directly in `-C linker`
(e.g. `-C linker=rust-lld`) often results in errors like:

```
cargo:warning=error: linking with `rust-lld` failed: exit status: 1ger, ppv-lite86, libc...
cargo:warning=  |
cargo:warning=  = note: LC_ALL="C" PATH="/home/vadorovsky/.rustup/toolchains/stable-x86_64-un
cargo:warning=  = note: rust-lld: error: unable to find library -lgcc_s
cargo:warning=          rust-lld: error: unable to find library -lc
cargo:warning=
cargo:warning=
cargo:warning=
cargo:warning=error: aborting due to 1 previous error
```

Not touching the linker settings is usually the best approach for Linux
systems. Native builds pick up the default C toolchain. Cross builds
default to GCC cross wrapper, but that's easy to supress with clang
and lld using RUSTFLAGS.

However, `-C linker=rust-lld` still works the best on macOS, where
Rust toolchains come with unwinder and runtime and there is usually no
need to link system libraries. Keep setting it only for macOS.

Fixes aya-rs#907

This way we are making sure that the integration tests infra doesn't
regress on musl environments.