Run unsupported platforms anyways, to hopefully improve platform coverage

[infinisil]

When a build doesn't eval due to unsupported platform, ofborg could run the build anyways, to see if it would succeed, if this were an allowed platform. In the reporting comment, it should then mention that:

Successfully built on darwin, even though it's not in meta.platforms

or

Failed to build on darwin, in accordance with this platform not being in meta.platforms

(or something along those lines)

This could improve the platform coverage situation.

[infinisil]

Caught in the wild: https://github.com/NixOS/nixpkgs/pull/43754#issuecomment-415961136

[Enzime]

For that PR more specifically, I'm not a trusted user so it doesn't run Darwin builds.

Potentially relevant issue: https://github.com/NixOS/ofborg/issues/204

[infinisil]

@Enzime Yeah, the first build however was a non-attempted darwin one: https://github.com/NixOS/nixpkgs/pull/43754#issuecomment-406035129