WIP: support building from Windows

[imjasonh]

Following up from https://github.com/chainguard-dev/apko/pull/536, this is a brief attempt to hide unix-isms behind Go build flags so that we can build apko for Windows.

~~There's one more, here, that I can't easily snip out:~~ https://github.com/chainguard-dev/apko/blob/89912f3e264800286f641cf86eeee367db1723ea/pkg/vfs/vfs.go#L34

edit: vfs.go appears to be unused by apko itself. Or at least, even with this not de-unix-ed, make apko passed on Windows. 🤔

If we think we care about Windows I think we're within throwing distance of it. If it's not worth it we can close this PR and carry this unspeakable horror to our graves.

[imjasonh]

With this change, the apko binary builds for Windows, but it fails to run on Windows:

Error: failed to build layer image: initializing apk: failed to initialize apk database: failed to create file /etc/apk/world: stat etc\apk: file does not exist
2023/02/24 22:02:08 error during command execution: failed to build layer image: initializing apk: failed to initialize apk database: failed to create file /etc/apk/world: stat etc\apk: file does not exist

https://github.com/imjasonh/apko/actions/runs/4266567359/jobs/7427240260