Miri CI broken by nightly-2026-02-12 regression in cargo #3499

Summary

The Miri CI workflow is failing on all branches since nightly-2026-02-12 with:

error: failed to run custom build command for `quote v1.0.44`

Caused by:
  process didn't exit successfully: `.../cargo-miri runner .../build-script-build` (exit status: 1)
  --- stderr
  fatal error: file ".../build-script-build" contains outdated or invalid JSON; try `cargo clean`

Root Cause

This is a confirmed upstream regression in cargo, not an issue with our code.

• Cause: rust-lang/cargo#16599 ("Add host.runner for wrapping host build target executions") was included in the cargo submodule update rust-lang/rust#152240 that shipped with nightly-2026-02-12. It accidentally applies host.runner to build script executions even when host-config is not enabled, causing cargo-miri to misinterpret build script binaries as JSON metadata.
• Upstream tracking: rust-lang/miri#4855 (filed by dtolnay)
• Fix: rust-lang/cargo#16631 is already merged in cargo. It will ship once the cargo submodule is updated in rust-lang/rust and a new nightly is published.

Evidence

Proposed Fix

Pin the nightly version in .github/workflows/miri.yml to nightly-2026-02-11 until the upstream fix ships in a new nightly:

       - name: Install Miri
         run: |
-          rustup toolchain install nightly --component miri
-          rustup override set nightly
+          rustup toolchain install nightly-2026-02-11 --component miri
+          rustup override set nightly-2026-02-11
           cargo miri setup

Once a fixed nightly is available, we should unpin back to nightly.