`incompatible_use_unstable_no_codegen_for_pipelining`

## Summary

`--@rules_rust//rust/settings:pipelined_compilation` currently runs `rustc --emit=metadata,link` and kills the process when the `.rmeta` artifact is written. This is fragile (races rustc's output), doesn't match Buck2's implementation, and leaves dep-info/incremental steps unrun.

The new implementation runs `rustc -Zno-codegen --emit=link=` to completion, producing a Buck2-style hollow rlib. Since `-Zno-codegen` is unstable, this requires `RUSTC_BOOTSTRAP=1` on the pipelined actions (set automatically; must be set on both metadata and full actions because it participates in SVH).

See PR #3870.

## Change

A new incompatible flag `incompatible_use_unstable_no_codegen_for_pipelining` has been introduced (defaulting to `false`) that switches the pipelining implementation to `-Zno-codegen`/hollow-rlib and sets `RUSTC_BOOTSTRAP=1` on pipelined actions.

Only has an effect when `pipelined_compilation` is also enabled.

## Migration

Opt in with:

```
--@rules_rust//rust/settings:incompatible_use_unstable_no_codegen_for_pipelining=true
```

If you cannot adopt `RUSTC_BOOTSTRAP=1`, leave this flag `false` until the old path is removed, or disable `pipelined_compilation`.