compiletest: forward disable-minification from bootstrap

[AayushMainali-Github]

build.docs-minification = false was already honored when building docs through bootstrap's doc steps, but compiletest-driven rustdoc suites always generated minified CSS/JS.

Bootstrap now forwards --disable-minification to compiletest when docs minification is disabled, and compiletest passes -Zunstable-options --disable-minification to rustdoc for HTML/JS/JSON/UI doc generation.

Fixes #142737.

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @oli-obk (or someone else) some time within the next two weeks.

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @jieyouxu, @oli-obk, @wesleywiser, bootstrap
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[oli-obk]

r? rustdoc

can individual tests override this flag? or... would they want to? is that useful?

[AayushMainali-Github]

No per-test override here. Tests can already add rustdoc flags via //@ doc-flags, but can't re-enable minification when bootstrap disables it. That seems fine for a global docs preference

[rust-bors]

☔ The latest upstream changes (presumably #160284) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.