Allow default platform to be specified with NERDCTL_DEFAULT_PLATFORM / DOCKER_DEFAULT_PLATFORM

### What is the problem you're trying to solve

I want to run x86 containers on macOS aarm64. Im faced with setting `platform: linux/amd64` in all my compose files and passing `--platform x86_64` on all my command line invocations. It'd be nice to set this in an env var in my profile or in a config file (probably the option for either is nice).

### Describe the solution you'd like

docker supports this with the DOCKER_DEFAULT_PLATFORM env var....could support that directly or simply have a dedicated nerdctl version: NERDCTL_DEFAULT_PLATFORM.

### Additional context

_No response_