fix: configure aarch64-unknown-linux-* cross-compilation

Sadly, simply installing `gcc-aarch64-linux-gnu` isn't quite enough to
cross-compile from `x86_64` linux to `aarch64`, we also need to:

- Override `cc` for the `aarch-64-unknown-linux-musl` target (there's no
  `gcc-aarch64-linux-musl` package).
- Override the linker for both targets (for some reason `cargo` doesn't
  apply the same default behaviour here as it does for `cc`).

.cargo/config.toml

@@ -0,0 +1,8 @@
+[env]
+CC_aarch64_unknown_linux_musl = "aarch64-linux-gnu-gcc"
+
+[target.aarch64-unknown-linux-gnu]
+linker = "aarch64-linux-gnu-gcc"
+
+[target.aarch64-unknown-linux-musl]
+linker = "aarch64-linux-gnu-gcc"