PoC: Add cross-compilation support

[szlend]

WIP PoC for #138. This is not ready for review, just a starting point. Though all the tests on my machine do seem to pass.

1. Wrap nix2container with makeScopeWithSplicing'. This gives us a way to express different platform variants of our builders/derivations (e.g. buildHost, hostTarget, etc).
2. Wrap all builders/derivations into the callPackage pattern. This enables package splicing so nativeBuildInputs/buildInputs automatically pick the correct platform variants of our derivations when cross-compiling.
3. (Optional) Since the callPackage pattern can be pretty noisy, I would suggest extracting individual builders/derivations to their own nix files where it makes sense. Similar to how crane does it here: https://github.com/ipetkov/crane/tree/master/lib

Happy to revert 3. if you feel this is going too far, but using callPackage in a single file is gonna add a lot of noise and additional indentation.

[szlend]

Had to move these to legacyPackages because they're not derivations.

[szlend]

This is the core change across most derivations. We reference nix2container-bin through nativeBuildInputs instead of ${nix2container-bin}/bin/nix2container. Paired with makeScopeWithSplicing/callPackage this automatically resolves to the correct variant of the package.

[szlend]

We explicitly set the image architecture here to go.GOARCH. This is the target architecture when cross-compiling.

[szlend]

We want these to run on the build platform. These need to be explicitly picked from the buildHost variant because it's not possible to automatically infer which variant we want here.

[szlend]

I added an alternative implementation that should be easier to review in #153