feat: Make Build Entrypoint configurable

[VJftw]

Adds an optional build_entrypoint field to the build_rule builtin to enable us to use binaries directly when building and support other cross-platform shells (e.g. nushell or Powershell).

TODO:

• Code docs, including design decisions/narration (e.g. where ExitOnError, etc. came from and what we could do in the future as a breaking change).
• PR description.

[peterebden]

Understood that this is still a WIP, but my initial take is: there's already an entry_points argument. Adding another called build_entrypoint needs some explanation of what it does vs. the existing one and why and how they differ.
I also think a dict with a bunch of 'magic' keys seems pretty messy. I feel like this wants to do something very specific - it would be useful to have a clearer description of the problem statement ("use binaries directly when building" seems like something that's already supported, and I'm uncertain how this would help to use something like Powershell).

[VJftw]

👋 I definitely should have started with an issue first! The TLDR is that i'm interested in removing the Bash dependency for build actions which is something Bazel supports via run_binary and it's also mentioned in their genrule doc.

In terms of implementation, I have the same concerns with the magic keys. I think it was the least amount of change/additions to make it somewhat work, and I got a little eager with the potential for using cross-platform shells like nushell which comes with a lot of built-in cross-platform commands.

Maybe a separate run_binary/run_binary_rule/binary_rule built-in would be a lot better/simpler to start with?

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Relevant comments/discussions which I think would benefit from this:

• Add support for suppressing SRCS env variables (in the case of rules with large numbers of sources) #3393 (comment)
• Spaces in root path cause plugins to fail #2863 (comment)
• Allow to define shell command by name and list of argument strings #3368