Add command run-custom-build.

[declantsien]

Add command run-custom-build for just run Cargo's build script.
See some context from #7178.

I left the doc/man changes for now.
Post the PR here hoping to receiving some feedback.

[rustbot]

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[ehuss]

Thanks for the PR! Considering that RFC 3344 has not yet been accepted, it may be premature to open a PR for the implementation. If there is some feedback you'd like to leave on the RFC, it would be helpful to leave a comment over there.

[declantsien]

Thanks for the PR! Considering that RFC 3344 has not yet been accepted,

Right, looking forward to that rfc.

Until then, I am working on this cargo-ninja. Generating build.ninja using data from Cargo build-plan.

Made it work for just run run-custom-build target for my project. So it's OK to wait a bit longer.

[epage]

Closing until we have an agreed-to-design.