Rename os_version to os.version

[orf]

I believe this should be os.version like ImageConfig?

For example this is what Github container repo returns:

{
  "schemaVersion": 2,
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:b09d1eb75170275f0e465ed97e90718e596aab4a2413ba7eedcfb9f13580aca6",
      "size": 2015,
      "platform": {
        "architecture": "amd64",
        "os": "darwin",
        "os.version": "macOS 13.3"
      },

Also the tests failed on MacOS. I fixed this by canonicalising the temporary directory as well:

thread 'runtime::tests::test_canonicalize_rootfs' panicked at 'assertion failed: `(left == right)`
  left: `"/var/folders/q1/c031nnz57ls4qm4dk3m3w74h0000gn/T/.tmpxAJ0q3/rootfs"`,
 right: `"/private/var/folders/q1/c031nnz57ls4qm4dk3m3w74h0000gn/T/.tmpxAJ0q3/rootfs"`', src/runtime/mod.rs:306:13

Unfortunately std::env::consts::OS returns macos instead of Darwin, so I added an extra arm to the match condition to handle this. This makes Default work properly on MacOS

[saschagrunert]

Can we avoid that print statement?

[orf]

Sorry, I forgot about this and deleted my fork. I've re-made it in #163