fix(compile): Make build.warnings ignore non-local deps

[epage]

What does this PR try to resolve?

This came up in rust-lang/rust#153809.
Considering we have the following, it seems reasonable to carry it
forward:

fn add_cap_lints(bcx: &BuildContext<'_, '_>, unit: &Unit, cmd: &mut ProcessBuilder) {
    // If this is an upstream dep we don't want warnings from, turn off all
    // lints.
    if !unit.show_warnings(bcx.gctx) {
        cmd.arg("--cap-lints").arg("allow");

    // If this is an upstream dep but we *do* want warnings, make sure that they
    // don't fail compilation.
    } else if !unit.is_local() {
        cmd.arg("--cap-lints").arg("warn");
    }
}

Part of #14802

How to test and review this PR?

The test runs against RUSTFLAGS=-Dwarnings as well to

• Help verify the test
• To clearly compare their behavior

[rustbot]

r? @weihanglo

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
• Random selection from ehuss, weihanglo

[Muscraft]

Thanks! I agree that it makes sense to match the existing cap-lints behaviour here.

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