Expose interface_library_builder in toolchain

[keith]

Without this you have to hack in the path to your actual tool in your
toolchain. Currently the script in this repo is unusable and also only
does cp

[armandomontanez]

Ideally, this is passed around through action configs. Exposing the tool this way seems like the wrong direction. I know this is a little frustrating.

My only initial idea is to pipe this through tool_paths from CcToolchainConfigInfo.

[keith]

I don't mind doing that! I think this case is particularly weird. In all of my other prs that touch stuff like this I've been trying to move away from using tool_paths (which as you know aren't even supported by the rules based toolchains #667), so moving something into that seems pretty off. But at the same time unlike the other "fake" actions we've added, like llvm_cov, llvm-profdata or gcov those could at least maybe be real looking actions in the future. Where in this case this tool is passed as a path to another tool that actually does the linking https://github.com/keith/rules_cc/blob/796ec7430f070ff965055265141bb01703c8d8f5/cc/private/toolchain/unix_cc_toolchain_config.bzl#L1167

seems to me like making that a full action so we could call get_tool_for_action would be a bit weird. but maybe that's still better than the alternatives.