ci: lock build-channel workflow installs

[JoshuaBatty]

Summary

Lock the build-channel workflow installs to the repository lockfile in the two workflows that install the helper binary:

• Publish Channel (nightly)
• Update Channel

Why

The manual Publish Channel (nightly) run failed on the Install build-channel script step before it ever reached channel publishing or deployment.

Failing run:

• https://github.com/FuelLabs/fuelup/actions/runs/24062679117

The workflow currently runs:

• cargo install --debug --path ./ci/build-channel

Because that install is not locked, Cargo re-resolves dependencies on the runner instead of honoring the repo lockfile. In the failing run it pulled newer icu_* 2.2.0 crates, which require Rust 1.86, while the workflow is pinned to Rust 1.85.0.

The repo lockfile already pins the same dependency family to compatible 1.5.x versions, so this is dependency drift during CI install rather than a true project-wide Rust floor bump.

Change

Add --locked to the cargo install --path ./ci/build-channel command in both workflows so CI uses the checked-in lockfile when installing the helper.

Validation

Validated locally in Docker with rust:1.85 against this checkout:

• cargo install --debug --path ./ci/build-channel failed and re-resolved to icu_* 2.2.0, which requires Rust 1.86
• cargo install --locked --debug --path ./ci/build-channel succeeded on the same toolchain

That matches the GitHub Actions failure and confirms --locked is the correct fix without widening the workflow toolchain unnecessarily.

[cursor]

PR Summary

Low Risk
Low risk: CI/workflow determinism change plus a test-only update to use a fixed nightly date; no production runtime logic is modified.

Overview
CI installs are now deterministic. Both Publish Channel (nightly) and Update Channel workflows add --locked to cargo install --path ./ci/build-channel, preventing dependency re-resolution drift on GitHub runners.

Tests no longer depend on yesterday’s nightly publish. Override-based test helpers in tests/testcfg/mod.rs switch from nightly-{yesterday()} to a fixed OVERRIDE_DATE archived nightly, and update related examples/usages accordingly.

^{Reviewed by Cursor Bugbot for commit 90e4bc7. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

LCOV of commit 90e4bc7 during CI #2320

Summary coverage rate:
  lines......: 86.9% (2515 of 2893 lines)
  functions..: 60.0% (375 of 625 functions)
  branches...: 66.0% (272 of 412 branches)

Files changed coverage rate: n/a