Pin dtolnay/rust-toolchain to master instead of stable

[glen-84]

Summary

• dtolnay/rust-toolchain publishes from master. Each toolchain branch (stable, beta, nightly, and one per Rust version) is a single commit on top of master that sets the default toolchain input, and is rebuilt whenever master moves. A pin taken from stable therefore soon belongs to no branch, and no newer version can be worked out from it.
• Both Rust steps in cost-oracle-nightly.yml now pin a master commit. Both steps already pass toolchain: stable, so they still install stable Rust.
• The new pin adds --force-non-host to the action's rustup toolchain install and rustup default calls. Nothing else in action.yml changes.

Test plan

• Confirmed with the GitHub compare API that the pinned commit is on master (behind).
• Reviewed the upstream diff from the previous pin's base to the new pin: action.yml changes by the two --force-non-host flags, plus a new job in the action's own CI.
• This workflow runs only on its schedule and on workflow_dispatch, so this PR's CI does not exercise it. The first nightly run after merge does, or a manual dispatch on this branch.

[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed workflow changes are focused and have no unresolved blocking issues.

Review effort: Lite
Findings: None

What changed in this PR

Updates the nightly cost-oracle workflow to pin dtolnay/rust-toolchain to a commit on master while continuing to install stable Rust.

Changes:

• Updated both Rust toolchain action pins.
• Preserved toolchain: stable in both jobs.

File	Description
.github/​workflows/​cost-oracle-nightly.yml	Updates both Rust installation steps to the new upstream commit.

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