more rigorously handle "skipping installers" situations

The installers code is riddled with checks for situations that make them invalid:

https://github.com/axodotdev/cargo-dist/blob/676214a910a97da55c3657aaff4b266a162dbaea/cargo-dist/src/tasks.rs#L1014-L1017

https://github.com/axodotdev/cargo-dist/blob/676214a910a97da55c3657aaff4b266a162dbaea/cargo-dist/src/tasks.rs#L1079-L1082

All of them produce warnings, allowing things to potentially progress in messed up situations. I made these all warnings so that `cargo dist build` run locally ("host mode") wouldn't be blocked by some installers not making sense on the current platform.

It would be nice if some of these could be hard errors when they should be (like `cargo dist plan`), and if they included more context for why things failed (super common for us to print a warning at the top about "inconsistent repository values", but it's easy to miss that the two warnings are related).

There's also this warning in init:

https://github.com/axodotdev/cargo-dist/blob/676214a910a97da55c3657aaff4b266a162dbaea/cargo-dist/src/init.rs#L432-L435

which is a little more subtle because it's genuinely "fine" for someone to not have repository set properly if all they care about is tarballs.

Start with the linked checks in cargo-dist/src/tasks.rs and the warning in cargo-dist/src/init.rs, then compare how cargo dist build, cargo dist plan, and cargo dist init should treat invalid installer situations. Identify which cases should be hard errors and how related warnings should provide context. Done means the handling is consistent without blocking valid host-mode or tarball-only workflows.

Written by the indexing model from the issue text.