Rollup merge of #139863 - fmease:simp-doctest-build-arg-passing, r=GuillaumeGomez

rustdoc: Replace unstable flag `--doctest-compilation-args` with a simpler one: `--doctest-build-arg`

Tracking issue: https://github.com/rust-lang/rust/issues/134172.
Context: https://github.com/rust-lang/rust/pull/137096#issuecomment-2776318800

Yeets the ad hoc shell-like lexer for 'nested' program arguments.
No FCP necessary since the flag is unstable.

I've chosen to replace `compilation` with `build` because it's shorter (you now need to pass it multiple times in order to pass many arguments to the doctest compiler, so it matters a bit) and since I prefer it esthetically.

**Issue**: Even though we don't process the argument passed to `--doctest-build-arg`, we end up passing it via an argument file (`rustc `@argfile`)` which delimits arguments by line break (LF or CRLF, [via](https://doc.rust-lang.org/rustc/command-line-arguments.html#path-load-command-line-flags-from-a-path)) meaning ultimately the arguments still get split which is unfortunate. Still, I think this change is an improvement over the status quo.

I'll update the tracking issue if/once this PR merges. I'll also add the (CR)LF issue to 'unresolved question'.

r? GuillaumeGomez
r? notriddle

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